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    <p>Supposedly, as Nawaz said, the assignment version <em>should</em> be faster on most platform. That's because <code>memcpy()</code> will copy byte by byte while the second version could copy 4 bytes at a time.</p> <p>As it's always the case, you should always profile applications to be sure that what you expect to be the bottleneck matches the reality.</p> <p><strong>Edit</strong><br> Same applies to dynamic array. Since you mention C++ you should use <code>std::copy()</code> algorithm in that case.</p> <p><strong>Edit</strong><br> This is code output for Windows XP with GCC 4.5.0, compiled with -O3 flag:</p> <pre><code>extern "C" void cpy(float* d, float* s, size_t n) { memcpy(d, s, sizeof(float)*n); } </code></pre> <p>I have done this function because OP specified dynamic arrays too. </p> <p>Output assembly is the following:</p> <pre><code>_cpy: LFB393: pushl %ebp LCFI0: movl %esp, %ebp LCFI1: pushl %edi LCFI2: pushl %esi LCFI3: movl 8(%ebp), %eax movl 12(%ebp), %esi movl 16(%ebp), %ecx sall $2, %ecx movl %eax, %edi rep movsb popl %esi LCFI4: popl %edi LCFI5: leave LCFI6: ret </code></pre> <p>of course, I assume all of the experts here knows what <code>rep movsb</code> means.</p> <p>This is the assignment version:</p> <pre><code>extern "C" void cpy2(float* d, float* s, size_t n) { while (n &gt; 0) { d[n] = s[n]; n--; } } </code></pre> <p>which yields the following code:</p> <pre><code>_cpy2: LFB394: pushl %ebp LCFI7: movl %esp, %ebp LCFI8: pushl %ebx LCFI9: movl 8(%ebp), %ebx movl 12(%ebp), %ecx movl 16(%ebp), %eax testl %eax, %eax je L2 .p2align 2,,3 L5: movl (%ecx,%eax,4), %edx movl %edx, (%ebx,%eax,4) decl %eax jne L5 L2: popl %ebx LCFI10: leave LCFI11: ret </code></pre> <p>Which moves 4 bytes at a time.</p>
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    1. POIn what cases should I use memcpy over standard operators in C++?
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    1. CO@Simone : the first para makes sense to me. Now I need to verify it, because I'm not sure. :-)
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    2. COI don;t think memcopy copies byte by byte. It is specifically designed to copy large chunks of memory very efficiently.
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      1. USMartin York
    3. COSource please? Only thing that POSIX mandates is [this](http://pubs.opengroup.org/onlinepubs/9699919799/functions/memcpy.html). BTW, see if [this implementation](http://www.gnu.org/software/mifluz/doc/doxydoc/memcpy2_8c-source.html) is that fast.
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