vector assign from pointer

• Stack Overflow Public questions & answers
• Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers
• Talent Build your employer brand
• Advertising Reach developers & technologists worldwide
• Labs The future of collective knowledge sharing
• About the company

Collectives™ on Stack Overflow

Find centralized, trusted content and collaborate around the technologies you use most.

Q&A for work

Connect and share knowledge within a single location that is structured and easy to search.

Assigning a vector from an array (pointer)

Is there a standard way to accomplish this that is better than a for loop?

If I had an array type supposedly I can do this:

But I can't do this when I only have a double * and an int indicating its length.

Edit: Actually, I think I actually can do this. The error messages are quite a handful, though, if you get your type parameters wrong (which is why it didn't work for me at first).

• Have you tried it? Can you show it to us? –  Some programmer dude Mar 1, 2012 at 4:41
• It seems to work now. When trying to do this with mis-matched types the error messages are incredibly verbose although not entirely cryptic. –  Steven Lu Mar 1, 2012 at 4:44
• You might wanna look at this question stackoverflow.com/questions/231491/… –  bisarch Mar 1, 2012 at 5:10

2 Answers 2

Of course you can do it the same way

Yep. You should be able to do this. The relevant constructor is template <class InputIterator> vector ( InputIterator first, InputIterator last, const Allocator& = Allocator() ); - and pointers are iterators too (since you can do integer arithmetic on them).

That being said, all that the vector is doing is iterating over the elements of d_array , so don't expect the performance to be significantly better than the for loop version.

• It's really quite cool that it's able to transparently use pointers like this. That this could in any way have better performance than the for loop version is incredible (though not really of course because a naive for loop would cause the vector to resize itself). Anyway, I like things like this because it helps cuts down on bugs. –  Steven Lu Mar 1, 2012 at 4:47
• It may be smart enough to subtract the two pointers to find out how many values it'll be looping over, and allocate enough space for them initially. Default-constructing a vector and inserting the values yourself may require it to reallocate as the vector grows. –  Wyzard Mar 1, 2012 at 4:49
• Right, in my naive implementation I needed to figure out whether i wanted to use resize or reserve ... Using the ctor or assign with pointers is just so much neater. –  Steven Lu Mar 1, 2012 at 4:52
• It's possible it might be smart enough to do a pre-allocation, but it's not guaranteed that it can even make an educated guess. It just takes an InputIterator, and for those there's no guarantee that the iteration is contiguous. You can create an InputIterator that jumps ahead an arbitrary amount - say, 4 * sizeof(double), whatever you want - though it would certainly be possible to optimize, it's a lot of special casing to do so. (though, if I were implementing the standard, I would make that "guess" by pre-allocating the delta and trusting it to resize if my guess was wrong...) –  Matt Mar 1, 2012 at 6:44

Your Answer

Sign up or log in, post as a guest.

Required, but never shown

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct .

Not the answer you're looking for? Browse other questions tagged c++ stl or ask your own question .

• The Overflow Blog
• What it’s like being a professional workplace bestie (Ep. 603)
• Featured on Meta
• Moderation strike: Results of negotiations
• Our Design Vision for Stack Overflow and the Stack Exchange network
• Temporary policy: Generative AI (e.g., ChatGPT) is banned
• Call for volunteer reviewers for an updated search experience: OverflowAI Search
• Discussions experiment launching on NLP Collective

Hot Network Questions

• Prostration during offerings?
• According to an online course, ribose and adenine can bond to make ATP. Is this true?
• How do you interpret "minimum insertation" of a quill stem?
• Spell date in Japanese
• How to terminate a soil drain pipe into the earth and avoid blockages
• A simpler or more concise way to divide the boundary of a polygon into equal arc lengths?
• What is the word for when a life event is coming to an end like a vacation and you feel an immense feeling of sadness due to not wanting to leave
• Uniqueness of the uniform distribution on hypersphere
• Is there a negative impact on an SSD drive if I power cycle it every hour for 6 months?
• I'm liking to take it easy on the weekend
• Vita Bene Vixit in a commemorative magazine
• Is tropylium cyclopentadienide possible?
• Coworker hiding and stealing lab material
• Is the liquid inside the canned chickpeas meant for consumption?
• What do Americans say instead of “can’t be bothered”?
• Why are radio telescopes often built into natural depressions?
• Why do programming languages use delimiters (quotes) for strings?
• Is it safe to create a public ID by hashing a private key?
• Could you find the way in this maze of rebuses?
• Reward flight cancelled and denied rerouting
• How to make scrollable TogglerBar?
• Does Bayesianism give an out for pseudoscience that it shouldn’t deserve?
• Causal expansion of EM fields
• Why did 1990s-2000s LCD all use 60 Hz refresh?

Your privacy

By clicking “Accept all cookies”, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy .

• Windows Programming
• UNIX/Linux Programming
• General C++ Programming
• Pointer to a vector

  Pointer to a vector

• C++ Data Types
• C++ Operators
• C++ Input/Output
• C++ Control Statements
• C++ Functions
• C++ Strings
• C++ Pointers
• C++ Memory Management
• C++ Exception Handling
• C++ Templates
• C++ Interview Questions
• Write an Interview Experience
• Share Your Campus Experience
• How does a vector work in C++?
• auto_ptr vs unique_ptr vs shared_ptr vs weak_ptr in C++
• Smart Pointers in C++
• Opaque Pointer in C++
• What is Dynamic Memory Allocation?
• Vector of Vectors in C++ STL with Examples
• vector :: cbegin() and vector :: cend() in C++ STL
• How to reverse a Vector using STL in C++?
• Using std::vector::reserve whenever possible
• std :: reverse_copy in C++ STL
• C++ – Pointer to Structure
• Count elements in a vector that match a target value or condition
• How to join two Vectors using STL in C++?
• How to find the maximum element of a Vector using STL in C++?
• Initialize a vector in C++ (7 different ways)
• vector::crend() & vector::crbegin() with example
• Passing a vector to constructor in C++
• Sorting a vector in C++
• Swapping of subranges from different containers in C++
• Vector in C++ STL
• Map in C++ Standard Template Library (STL)
• std::sort() in C++ STL
• Bitwise Operators in C/C++
• The C++ Standard Template Library (STL)
• Inheritance in C++
• Object Oriented Programming in C++
• C++ Classes and Objects
• Set in C++ Standard Template Library (STL)

C++ Vector of Pointers

Prerequisites

• Pointers in C++
• Vectors in C++

Vector of pointers are vectors that can hold multiple pointers. Each pointer within a vector of pointers points to an address storing a value. We can use the vector of pointers to manage values that are not stored in continuous memory.

How to Create Vector of Pointers in C++?

Similar to any other vector declaration we can declare a vector of pointers. In C++ we can declare vector pointers using 3 methods:

•  Using std::vector container 
•  Using [ ] notations  
•  Using the new keyword (Dynamic Memory)

1. Using std::vector container 

Using vectors to create vector pointers is the easiest and most effective method as it provides extra functionality of STL.

A. Insert elements in a vector:

  We can insert elements by 2 methods:

• while initialization
• using push_back( )

Insertion while initialization: Although it’s an option that can be used we should avoid such type of insertion as vectors store addresses within them.

Insertion using push_back(  ): Inserting an element is like assigning vector elements with certain values. We can perform this task in certain steps.

• Create a variable and insert a value in it. 
• Insert the address of the variable inside the vector.

B. Deletion of Elements

Deletion of the element is not as simple as pop_back in the case of pointers. It can be done using 2 steps:

• Free the pointer (Remove address from variable)
• Erase the variable.

2. Using [ ] notations 

Square brackets are used to declare fixed size. This can be used to operate over to create an array containing multiple pointers. This is a type of array that can store the address rather than the value. So, can be called a pointer array, and the memory address is located on the stack memory rather than the heap memory.

3. Using the new keyword 

The new Keyword in C++ represents dynamic memory allocation i.e, heap memory. The code will suffer from a memory leak if the programmer does not free up the memory before exiting. This can lead to a huge problem in long-running applications or resource-constrained hardware environments.

Please Login to comment...

Improve your coding skills with practice.

Stack Exchange Network

Stack Exchange network consists of 183 Q&A communities including Stack Overflow , the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

Software Engineering Stack Exchange is a question and answer site for professionals, academics, and students working within the systems development life cycle. It only takes a minute to sign up.

Q&A for work

Connect and share knowledge within a single location that is structured and easy to search.

C++: Is a pointer to a vector going to cause memory issues?

I started to write a function which had a pointer to a vector as a parameter so that it could modify that vector to output results (as the actual return value was an error code), when I started to think about the memory behind that.

Edit: Adding a new example here to make this clearer - I think I mostly have the idea from the answers already posted, but my example was poor in that it was not a good example of why to use a pointer - and did not exactly exhibit the properties I wanted.

(New example)

I am aware that I could use a reference instead of a pointer to output_vect, but the linter I was using got mad at me for this, so I'm evaluating whether a pointer makes more sense.

(Old example) For example, if I have the following code:

And I run it, I get the following output:

It seems as though this could cause major memory issues - because if the vector needs to expand, it could be writing into space which is already being utilized, because it looks like that pointer does not change. Even running this over a much larger loop, this pointer looks like it is constant.

I'm still a (relatively) new programmer, and am not sure that I have the grasp on memory allocation that I would like to. Is my understanding correct - will this cause buffer errors and overwrite adjacent memory? Or is there some protection in std::vector that I am not considering?

(Edit): From some of the answers below, it appears that a layer of indirection between the pointer to the vector and where the elements of the vector are stored were what I was not accounting for. However, since the answers seemed to focus on how I allocated the vector, when I don't plan to use the new operator at all (just call the function like my_obj.MyFunction((other input parameters), &output_vect) ), I wanted to make this more clear.

• 1 I recommend to learn this in two steps: first learn how the memory allocation of a std::vector works internally . Second, you need to learn the difference between new for a single object and new for arrays . –  Doc Brown Apr 10, 2021 at 5:17
• 2 ... Finally you need to understand that you mixed up the latter with what vector does internally - allocating a single vector object is what your code does, and it has absolutely nothing to do with the memory allocation which happens inside the vector object. –  Doc Brown Apr 10, 2021 at 5:21
• 1 This works because you're pointing to the vector object itself; if instead you had a direct pointer to one of the elements, after relocation they could end up pointing to gibberish. –  Filip Milovanović Apr 10, 2021 at 12:31

4 Answers 4

No problems luckily. This code is safe.

When calling “new”, a range of bytes is reserved on the heap memory. The size of that range depends on the object type and is known at compile time. That size can not grow or shrink, until you call delete, and you can count on that pointer value not to change (until you call delete).

But then, how can this vector “grow” when you push more elements on it?

The answer is: through indirection :

The vector object itself might have a fixed allocated size (24 bytes in my case, but that depends). In that space it holds its own pointer(s) to extra memory. Those might get deleted and renewed when the vector is required to grow, and so that pointer will get a different value. But all of that is hidden from the user of the vector: you.

See also this question: https://stackoverflow.com/questions/34024805/c-sizeof-vector-is-24

While this code works, the question is why do you want a pointer to a vector? If this pointer is a class member, extra care is required with a view to the rule of three . If this is a local variable somewhere, extra care is needed to avoid leaking memory when the pointer is no longer needed.

The following code is much safer:

There seem to be no need to use a pointer at all in your case. Move semantics on vectors may generate code as efficient as pointers gymnastics but much safer . And there is no issue with memory leaks or dangling pointer at all.

In modern C++, the use of pointers with manual memory management (i.e. ownership of objects) should be an exception that needs to be duly justified. And if pointers are needed, smart pointers should be preferred.

• @Deduplicator indeed! Thanks for the suggestion; I've edited to make this clearer. –  Christophe Apr 10, 2021 at 10:38
• Thank you very much, @Christophe. To be fair, the example I coded was nothing like what I was trying to do - it was just for helping me to see where memory was and see what happened. I 100% agree that there was no need for a pointer there. The reason for my initial attempt to use this was that I needed a function of the form ErrorEnum Foo(...., std::vector<StructType> &output_vect) , but running my code through the linter failed because output_vect was not a constant reference. So, I started to turn it into a pointer, and then fell down a rabbit-hole of "do I really know the consequences?" –  I_like_robots Apr 10, 2021 at 18:38
• 1 @I_like_robots I'm not sure what linter you're using but sometimes the best course of action is to ignore it. Linters are not inerrant; sometimes they get confused about what you're trying to do. (Admittedly I also don't really understand the constraints here, so maybe there's a better alternative) –  trent Apr 12, 2021 at 15:21
• 2 @I_like_robots If it’s an output reference, there is no reason for the linter to require a const. Does the compiler complain too? My advice: do not change a type just to please a linter or a compiler, without understanding why it does not accept what you want. I’ve seen the weirdest bugs on SO caused exactly by this approach. –  Christophe Apr 12, 2021 at 18:41

A little bit more test code helped me to understand the great answers above, for anyone who may still be confused.

This gave me the output

Interpreting this shows that, as @Kris Van Bael states, there is a layer of indirection between the vector and its elements - specifically, I had made the false assumption that a vector worked like an array - that the address of the vector was also the address of the first element. This is clearly not the case - the location in memory at which the vector is stored is NOT the same location at which its data elements are stored.

So, when the vector needs to grow or shrink, there is no need for its address to change - it just moves the pointer to its allocated data memory. It's pretty obvious this is necessary, but I was not considering the differences between an array and a vector.

When you have a class where instances could store arbitrary amounts of data, like vector or string or a map, the class instances will not contain the actual data, but just pointers to the data. So the instance itself never changes in size as you add things, only the private pointers inside it.

As a rule, the pointer to an object never changes.

Your Answer

Sign up or log in, post as a guest.

Required, but never shown

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct .

Not the answer you're looking for? Browse other questions tagged c++ memory pointers or ask your own question .

• The Overflow Blog
• What it’s like being a professional workplace bestie (Ep. 603)
• Featured on Meta
• Moderation strike: Results of negotiations
• Our Design Vision for Stack Overflow and the Stack Exchange network

Hot Network Questions

• Batch Render 300 Objects
• Agreement of subject and verb in Luke 11:41
• Uniqueness of the uniform distribution on hypersphere
• Has anyone been charged with a crime committed in space?
• Is tropylium cyclopentadienide possible?
• Is wearing a mask in public a no-no (making one looks weird)?
• How much monthly costs can I expect if I get a kitten or two?
• How would you say "I can't help but wonder whether..." in Latin?
• Someone I don't know contributed to my program on GitHub. Can I still present that program as my Bachelor thesis?
• How to force a resource pack on a world?
• Is there a negative impact on an SSD drive if I power cycle it every hour for 6 months?
• Did Einstein say "Do not worry about your difficulties in mathematics, I assure you that mine are greater"?
• Why do people say 'topless' but not 'topful'?
• Pros and cons to round PCB design
• How to properly define volume for beginner calculus students?
• Causal expansion of EM fields
• Best practices for community choir audition with unknown song
• An open-source license that seems impossible to comply with
• Correcting how a student writes symbols
• Coworker hiding and stealing lab material
• Does "assess the use of two strategies by this business as an effective strategy" mean that both strategies have to be effective, or is it ambiguous?
• Could you find the way in this maze of rebuses?
• PNG files in photography workflow
• Indispensable, Essential, "Tool of the trade", "Staple item"

Your privacy

By clicking “Accept all cookies”, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy .

• C++ Tutorial
• Language Basics
• Operators statements
• Development
• Operator Overloading
• File Stream
• STL Introduction
• set multiset
• queue stack
• map multimap
• STL Algorithms Modifying sequence operations
• STL Algorithms Non modifying sequence operations
• STL Algorithms Binary search
• STL Algorithms Sorting
• STL Algorithms Merge
• STL Algorithms Min Max
• STL Algorithms Iterator
• STL Algorithms Heap
• STL Algorithms Helper

Change element with pointer : vector elements « vector « C++ Tutorial

• vector elements

C++ Cookbook by D. Ryan Stephens, Christopher Diggins, Jonathan Turkanis, Jeff Cogswell

Get full access to C++ Cookbook and 60K+ other titles, with a free 10-day trial of O'Reilly.

There are also live events, courses curated by job role, and more.

6.4. Storing Pointers in a vector

For efficiency or other reasons, you can’t store copies of your objects in a vector , but you need to keep track of them somehow.

Store pointers to your objects in a vector instead of copies of the objects themselves. But if you do, don’t forget to delete the objects that are pointed to, because the vector won’t do it for you. Example 6-4 shows how to declare and work with vector s of pointers.

Example 6-4. Using vectors of pointers

You can store pointers in a vector just like you would anything else. Declare a vector of pointers like this:

The important thing to remember is that a vector stores values without regard for what those values represent. It, therefore, doesn’t know that it’s supposed to delete pointer values when it’s destroyed. If you allocate memory, then put pointers to that ...

Get C++ Cookbook now with the O’Reilly learning platform.

O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.

Don’t leave empty-handed

Get Mark Richards’s Software Architecture Patterns ebook to better understand how to design components—and how they should interact.

It’s yours, free.

Check it out now on O’Reilly

Dive in for free with a 10-day trial of the O’Reilly learning platform—then explore all the other resources our members count on to build skills and solve problems every day.

cppreference.com

• View source

std:: vector

The elements are stored contiguously, which means that elements can be accessed not only through iterators, but also using offsets to regular pointers to elements. This means that a pointer to an element of a vector may be passed to any function that expects a pointer to an element of an array.

The storage of the vector is handled automatically, being expanded as needed. Vectors usually occupy more space than static arrays, because more memory is allocated to handle future growth. This way a vector does not need to reallocate each time an element is inserted, but only when the additional memory is exhausted. The total amount of allocated memory can be queried using capacity() function. Extra memory can be returned to the system via a call to shrink_to_fit() [1] .

Reallocations are usually costly operations in terms of performance. The reserve() function can be used to eliminate reallocations if the number of elements is known beforehand.

The complexity (efficiency) of common operations on vectors is as follows:

• Random access - constant 𝓞(1) .
• Insertion or removal of elements at the end - amortized constant 𝓞(1) .
• Insertion or removal of elements - linear in the distance to the end of the vector 𝓞(n) .

std::vector (for T other than bool ) meets the requirements of Container , AllocatorAwareContainer (since C++11) , SequenceContainer , ContiguousContainer (since C++17) and ReversibleContainer .

• ↑ In libstdc++, shrink_to_fit() is not available in C++98 mode.

Template parameters

[edit]

Specializations

The standard library provides a specialization of std::vector for the type bool , which may be optimized for space efficiency.

Iterator invalidation

Member types, member functions, non-member functions, defect reports.

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

• Recent changes
• Offline version
• What links here
• Related changes
• Upload file
• Special pages
• Printable version
• Permanent link
• Page information
• In other languages
• This page was last modified on 14 August 2023, at 06:04.
• This page has been accessed 13,271,060 times.
• Privacy policy
• About cppreference.com
• Disclaimers

Help Center Help Center

• Help Center
• Trial Software
• Product Updates
• Documentation

Convert Data Copies to Pointer Assignments

The code generator optimizes generated code for vector signal assignments by trying to replace for loop controlled element assignments and memcpy function calls with pointer assignments. Pointer assignments avoid expensive data copies. Therefore, they use less stack space and offer faster execution speed than for loop controlled element assignments and memcpy function calls. If you assign large data sets to vector signals, this optimization can result in significant improvements to code efficiency.

Configure Model to Optimize Generated Code for Vector Signal Assignments

To apply this optimization:

Verify that your target supports the memcpy function.

Determine whether your model uses vector signal assignments (such as Y=expression ) to move large amounts of data. For example, your model could use a Selector block to select input elements from a vector, matrix, or multidimension signal.

On the Optimization pane, the Use memcpy for vector assignment parameter , which is on by default, enables the associated Memcpy threshold (bytes) parameter.

Examine the setting of Memcpy threshold (bytes) . By default, it specifies 64 bytes as the minimum array size for which memcpy function calls or pointer assignments can replace for loops in the generated code. Based on the array sizes in your application's vector signal assignments, and target environment considerations on the threshold selection, accept the default value or specify another array size.

Example Model

Open the example model rtwdemo_pointer_conversion . This model uses a Switch block to assign data to a vector signal. This signal then feeds into a Bus Selector block.

Generate Code Without Optimization

In the Configuration Parameters dialog box, clear the Use memcpy for vector assignment parameter. Alternatively, use the command-line.

Press Ctrl+B to generate code. Alternatively, use the command-line.

View the generated code without the optimization. Here is a portion of rtwdemo_pointer_conversion.c .

Without the optimization, the generated code contains for loop controlled element assignments.

Enable Optimization and Generate Code

In the Configuration Parameter dialog box, select the Use memcpy for vector assignment parameter.

Generate code.

Because the setting of the Memcpy threshold (bytes) parameter is below the array sizes in the generated code, the optimized code contains pointer assignments for the vector signal assignments.

Use memcpy for vector assignment | Memcpy threshold (bytes)

Related Topics

• Design Techniques to Optimize Models for Efficient Code Generation
• Use memcpy Function to Optimize Generated Code for Vector Assignments
• Vector Operation Optimization

Open Example

You have a modified version of this example. Do you want to open this example with your edits?

MATLAB Command

You clicked a link that corresponds to this MATLAB command:

Run the command by entering it in the MATLAB Command Window. Web browsers do not support MATLAB commands.

Select a Web Site

Choose a web site to get translated content where available and see local events and offers. Based on your location, we recommend that you select: .

• Switzerland (English)
• Switzerland (Deutsch)
• Switzerland (Français)
• 中国 (English)

You can also select a web site from the following list:

How to Get Best Site Performance

Select the China site (in Chinese or English) for best site performance. Other MathWorks country sites are not optimized for visits from your location.

• América Latina (Español)
• Canada (English)
• United States (English)
• Belgium (English)
• Denmark (English)
• Deutschland (Deutsch)
• España (Español)
• Finland (English)
• France (Français)
• Ireland (English)
• Italia (Italiano)
• Luxembourg (English)
• Netherlands (English)
• Norway (English)
• Österreich (Deutsch)
• Portugal (English)
• Sweden (English)
• United Kingdom (English)

Asia Pacific

• Australia (English)
• India (English)
• New Zealand (English)

Contact your local office