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copied!<p>It depends on what you are comparing.</p> <p>Your benchmark measures both <em>setup</em> time and <em>access</em> times together. It's doubtless that <code>std::vector</code> has a more expensive setup time. This is because it needs to allocate memory, and then (by necessity of the standard) call default constructors on all the elements. Which for a POD type, means zeroing.</p> <p>So if you're trying to measure access times, then no your benchmark isn't accurate.</p> <p>Here's some numbers to digest:</p> <p><strong>Original Code:</strong></p> <pre><code>StartCounter(); vector<int> test1(vectorsize); for(int i=0; i<vectorsize; i++){ test1[i] = 5; } cout << GetCounter() << endl << endl; </code></pre> <p><strong>Time: 444353.5206</strong></p> <hr> <p>Start timing <em>after</em> declaring and initializing the <code>vector</code>:</p> <pre><code>vector<int> test1(vectorsize); StartCounter(); for(int i=0; i<vectorsize; i++){ test1[i] = 5; } cout << GetCounter() << endl << endl; </code></pre> <p><strong>Time: 15031.76101</strong></p> <hr> <p>And for the array:</p> <pre><code>StartCounter(); int test2[vectorsize]; for(int i=0; i<vectorsize; i++){ test2[i] = 5; } cout << GetCounter() << endl << endl; </code></pre> <p><strong>Time: 38129.345</strong></p> <p>The times are about the same regardless of whether the declaration is timed. This is likely because stack allocation is done all at once upon entry to the function.</p> <hr> <p>Basically, the vector memory allocation and initialization is taking a disproportionate amount of time. But the actual loop is fast.</p> <p>I'll also note that your current benchmark framework is still sightly flawed. You only make one pass over each array. So cache-effects and lazy-allocation will be significant.</p> <p>The reason why the array is now slower is likely due to lazy-allocation. The array is allocated, but it hasn't been committed yet. Lazy allocation means that it is committed upon first access - which involves a page-fault and a context-switch to the kernel.</p> <hr> <p>Here's a fairer test with an outer loop to increase the benchmark time:</p> <pre><code>vector<int> test1(vectorsize); StartCounter(); for (int c = 0; c < 10000; c++){ for(int i=0; i<vectorsize; i++){ test1[i] = 5; } } cout << GetCounter() << endl << endl; </code></pre> <p><strong>Time: 227330454.6</strong></p> <pre><code>int test2[vectorsize]; memset(test2,0,sizeof(test2)); StartCounter(); for (int c = 0; c < 10000; c++){ for(int i=0; i<vectorsize; i++){ test2[i] = 5; } } cout << GetCounter() << endl << endl; cout << test2[0]; </code></pre> <p><strong>Time: 212286228.2</strong></p> <p>So no an array is <strong><em>NOT</em></strong> faster than a vector for steady-state access. It's just tricky to benchmark properly.</p>

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