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POHow can I get the address of the buffer allocated by vector::reserve()?
 

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  1. POHow can I get the address of the buffer allocated by vector::reserve()?
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    <p>I have a std::vector of values for which I know the maximum size, but the actual size will vary during usage:</p> <pre><code>void setupBuffer(const size_t maxSize) { myVector.reserve(maxSize); } void addToBuffer(const Value&amp; v) { myVector.push_back(v); if (myVector.size() == maxSize) { // process data... myVector.clear(); } } </code></pre> <p>However, in setupBuffer, I need to obtain a pointer to the start of myVector's data. I'm using a third party library where I must cache this pointer up front for use in a call made during the "process data..." section.</p> <pre><code>void setupBuffer(const size_t maxSize) { myVector.reserve(maxSize); cachePtr(&amp;(myVector[0])); // doesn't work, obviously } </code></pre> <p>I don't want to resize() the vector up front, as I want to use vector.size() to mean the number of elements added to the vector.</p> <p>So, is there any way to obtain the pointer to the vector's buffer after allocation (reserve()) but before it has any elements? I would imagine the buffer exists (and won't move as long as I restrict the number of push_back'd values)....maybe this isn't guaranteed?</p>
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    1. USRob Kennedy
    1. USRyan
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    1. COWhy do you need to cache the pointer, rather than safely reading it when you need it?
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      1. POHow can I get the address of the buffer allocated by vector::reserve()?
      1. USMike Seymour
    2. COI'm using a 3rd-party library for I/O to a specific file format. The way it works is to set up 'buffers' in the library by sending a pointer and max size. Then you can asynchronously call e.g. 'write' which flushes the data to disk. I could set up the pointer each time I hit maxSize, but this would be additional overhead (how much I'd have to test). Good point, though.
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      1. POHow can I get the address of the buffer allocated by vector::reserve()?
      1. USRyan
    3. CONote that this question asks how to get the pointer, but answers differ on what you're allowed to *do* with that pointer once you have it. For a question that covers that issue directly, please see [Is accessing the raw pointer after std::vector::reserve safe?](http://stackoverflow.com/questions/8228072/is-accessing-the-raw-pointer-after-stdvectorreserve-safe)
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      1. USRob Kennedy
 

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