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copied!<p>Yes, the compiler will almost certainly do something like a memcpy to copy the struct or class of hundreds of bytes onto the stack if that's what you asked for. If that wasn't the case something like this wouldn't work:</p> <pre><code>std::string s = "A large amount of text"; std::string r = rev(s); std::cout << s << " reversed is " << r << std::endl; ... std::string rev(std::string s) { std::string::size_type len = s.length(); for(std::string::size_type i = 0; i < len / 2; i++) { swap(s[i], s[len-i]); } return s; } </code></pre> <p>This is why it's nearly always recommended to use <code>const</code> references when possible, as it passes just a pointer to the object.</p> <p>Since the above example got objected to, here's another example:</p> <pre><code>class mystring { char s[200]; size_t len; public: mystring(const char *aS) { strcpy(s, aS); len = strlen(s); } char& operator[](int index) { return s[index]; } size_t length() { return len; } } mystring str("Some long string"); mystring rev = rev_my_str(s); mystring rev_my_str(mystring s) { size_t len = s.length(); for(size_t i = 0; i < len / 2; i++) { swap(s[i], s[len-i]); } return s; } </code></pre> <p>In fact, this will make space for TWO <code>mystring</code> objects on the stack, one for <code>s</code> going into <code>rev_my_str</code>, and one for the return value.</p> <p>Edit:</p> <p>Assembler generated by <code>g++ -O1</code> [1] for the call to <code>rev_my_string</code> as above. The interesting bit is the <code>rep movsq</code> along with the setup of <code>%ecx</code>, <code>%rsi</code> and <code>%rdi</code> (count, source and destination, respectively). $26 is the number of 8 byte units that it will copy. 26 * 8 = 208 bytes. <code>%rsp</code> is the stack pointer. This is almost exactly how a <code>memcpy</code> would look if it was inlined in a simple form [actual <code>memcpy</code> most likely has a whole bunch of extra work to deal with unaligned start/end and using SSE instructions, etc]. </p> <pre><code>movl $26, %ecx movq %rsp, %rdi movq %rbx, %rsi rep movsq leaq 416(%rsp), %rdi call _Z10rev_my_str8mystring </code></pre> <p>And rev_my_string itself looks like this. Note the <code>rep movsq</code> at the bottom of the function. That's where it stores back the resulting string.</p> <pre><code> _Z10rev_my_str8mystring: .LFB990: .cfi_startproc movq %rdi, %rax movq 208(%rsp), %r9 movq %r9, %r10 shrq %r10 je .L5 addq $1, %r10 movl $1, %edx .L6: movl %r9d, %ecx subl %edx, %ecx leaq 7(%rsp), %rsi addq %rdx, %rsi movzbl (%rsi), %edi movslq %ecx, %rcx movzbl 8(%rsp,%rcx), %r8d movb %r8b, (%rsi) movb %dil, 8(%rsp,%rcx) addq $1, %rdx cmpq %r10, %rdx jne .L6 .L5: movl $26, %ecx movq %rax, %rdi leaq 8(%rsp), %rsi rep movsq ret </code></pre> <p>[1] Using higher optimisation than that makes the compiler inline too much of the code (for example the rev_my_string function gets inlined), and it gets very hard to see what goes on. </p>

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