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POReading and writing C++ vector to a file
 

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  1. POReading and writing C++ vector to a file
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    <p>For some graphics work I need to read in a large amount of data as quickly as possible and would ideally like to directly read and write the data structures to disk. Basically I have a load of 3d models in various file formats which take too long to load so I want to write them out in their "prepared" format as a cache that will load much faster on subsequent runs of the program.</p> <p>Is it safe to do it like this? My worries are around directly reading into the data of the vector? I've removed error checking, hard coded 4 as the size of the int and so on so that i can give a short working example, I know it's bad code, my question really is if it is safe in c++ to read a whole array of structures directly into a vector like this? I believe it to be so, but c++ has so many traps and undefined behavour when you start going low level and dealing directly with raw memory like this.</p> <p>I realise that number formats and sizes may change across platforms and compilers but this will only even be read and written by the same compiler program to cache data that may be needed on a later run of the same program.</p> <pre><code>#include &lt;fstream&gt; #include &lt;vector&gt; using namespace std; struct Vertex { float x, y, z; }; typedef vector&lt;Vertex&gt; VertexList; int main() { // Create a list for testing VertexList list; Vertex v1 = {1.0f, 2.0f, 3.0f}; list.push_back(v1); Vertex v2 = {2.0f, 100.0f, 3.0f}; list.push_back(v2); Vertex v3 = {3.0f, 200.0f, 3.0f}; list.push_back(v3); Vertex v4 = {4.0f, 300.0f, 3.0f}; list.push_back(v4); // Write out a list to a disk file ofstream os ("data.dat", ios::binary); int size1 = list.size(); os.write((const char*)&amp;size1, 4); os.write((const char*)&amp;list[0], size1 * sizeof(Vertex)); os.close(); // Read it back in VertexList list2; ifstream is("data.dat", ios::binary); int size2; is.read((char*)&amp;size2, 4); list2.resize(size2); // Is it safe to read a whole array of structures directly into the vector? is.read((char*)&amp;list2[0], size2 * sizeof(Vertex)); } </code></pre>
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    1. USPeter Alexander
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    1. COTry to avoid using magic constants: `os.write( &size1, sizeof(size1) )` is better than hardcoding the 4 there. The same goes for the read.
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      1. USDavid Rodríguez - dribeas
    2. CO@David, try to avoid not reading the question before making comments ;)
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      1. USPeter Alexander
    3. CO@Poita_ :) I know that the change has been only for compaction, but the fact is that `4` is just slightly smaller than `sizeof(int)`, and the later should always be preferred, even in synthesized code snippets.
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      1. USDavid Rodríguez - dribeas
 

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