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POHow can I avoid std::vector<> to initialize all its elements?
 

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  1. POHow can I avoid std::vector<> to initialize all its elements?
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    <p><strong>EDIT:</strong> I edited both the question and its title to be more precise.</p> <p>Considering the following source code:</p> <pre><code>#include &lt;vector&gt; struct xyz { xyz() { } // empty constructor, but the compiler doesn't care xyz(const xyz&amp; o): v(o.v) { } xyz&amp; operator=(const xyz&amp; o) { v=o.v; return *this; } int v; // &lt;will be initialized to int(), which means 0 }; std::vector&lt;xyz&gt; test() { return std::vector&lt;xyz&gt;(1024); // will do a memset() :-( } </code></pre> <p>...how can I avoid the memory allocated by the vector&lt;> to be initialized with copies of its first element, which is a O(n) operation I'd rather skip for the sake of speed, since my default constructor does nothing ?</p> <p>A g++ specific solution will do, if no generic one exists (but I couldn't find any attribute to do that).</p> <p><strong>EDIT</strong>: generated code follows (command line: arm-elf-g++-4.5 -O3 -S -fno-verbose-asm -o - test.cpp | arm-elf-c++filt | grep -vE '^[[:space:]]+[.@].*$' )</p> <pre><code>test(): mov r3, #0 stmfd sp!, {r4, lr} mov r4, r0 str r3, [r0, #0] str r3, [r0, #4] str r3, [r0, #8] mov r0, #4096 bl operator new(unsigned long) add r1, r0, #4096 add r2, r0, #4080 str r0, [r4, #0] stmib r4, {r0, r1} add r2, r2, #12 b .L4 @ .L8: @ add r0, r0, #4 @ .L4: @ cmp r0, #0 @ fill the memory movne r3, #0 @ strne r3, [r0, #0] @ cmp r0, r2 @ bne .L8 @ str r1, [r4, #4] mov r0, r4 ldmfd sp!, {r4, pc} </code></pre> <p><strong>EDIT:</strong> For the sake of completeness, here is the assembly for x86_64:</p> <pre><code>.globl test() test(): LFB450: pushq %rbp LCFI0: movq %rsp, %rbp LCFI1: pushq %rbx LCFI2: movq %rdi, %rbx subq $8, %rsp LCFI3: movq $0, (%rdi) movq $0, 8(%rdi) movq $0, 16(%rdi) movl $4096, %edi call operator new(unsigned long) leaq 4096(%rax), %rcx movq %rax, (%rbx) movq %rax, 8(%rbx) leaq 4092(%rax), %rdx movq %rcx, 16(%rbx) jmp L4 @ L8: @ addq $4, %rax @ L4: @ testq %rax, %rax @ memory-filling loop je L2 @ movl $0, (%rax) @ L2: @ cmpq %rdx, %rax @ jne L8 @ movq %rcx, 8(%rbx) movq %rbx, %rax addq $8, %rsp popq %rbx leave LCFI4: ret LFE450: EH_frame1: LSCIE1: LECIE1: LSFDE1: LASFDE1: LEFDE1: </code></pre> <p><strong>EDIT:</strong> I think the conclusion is to not use <code>std::vector&lt;&gt;</code> when you want to avoid unneeded initialization. I ended up unrolling my own templated container, which performs better (and has specialized versions for neon and armv7).</p>
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      1. POHow can I avoid std::vector<> to initialize all its elements?
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    1. COPurely out of curiosity: Why?
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    2. COg++ does not auto-initialize values to 0. Some compilers do that in debug mode, but not in optimized mode.
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      1. POHow can I avoid std::vector<> to initialize all its elements?
      1. USRomain Hippeau
    3. CO@ildjarn: because filling memory with zeroes has a cost, and I'd like the compiler to respect my choice when I leave a value non initializaed.
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