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PODual emission of constructor symbols
 

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    <p>Today, I discovered a rather interesting thing about either <code>g++</code> or <code>nm</code>...constructor definitions appear to have two entries in libraries.</p> <p>I have a header <code>thing.hpp</code>:</p> <pre><code>class Thing { Thing(); Thing(int x); void foo(); }; </code></pre> <p>And <code>thing.cpp</code>:</p> <pre><code>#include "thing.hpp" Thing::Thing() { } Thing::Thing(int x) { } void Thing::foo() { } </code></pre> <p>I compile this with:</p> <pre><code>g++ thing.cpp -c -o libthing.a </code></pre> <p>Then, I run <code>nm</code> on it:</p> <pre><code>%&gt; nm -gC libthing.a 0000000000000030 T Thing::foo() 0000000000000022 T Thing::Thing(int) 000000000000000a T Thing::Thing() 0000000000000014 T Thing::Thing(int) 0000000000000000 T Thing::Thing() U __gxx_personality_v0 </code></pre> <p>As you can see, both of the constructors for <code>Thing</code> are listed with two entries in the generated static library. My <code>g++</code> is 4.4.3, but the same behavior happens in <code>clang</code>, so it isn't just a <code>gcc</code> issue.</p> <p>This doesn't cause any apparent problems, but I was wondering:</p> <ul> <li>Why are defined constructors listed twice?</li> <li>Why doesn't this cause "multiple definition of symbol __" problems?</li> </ul> <hr> <p><strong>EDIT</strong>: For Carl, the output without the <code>C</code> argument:</p> <pre><code>%&gt; nm -g libthing.a 0000000000000030 T _ZN5Thing3fooEv 0000000000000022 T _ZN5ThingC1Ei 000000000000000a T _ZN5ThingC1Ev 0000000000000014 T _ZN5ThingC2Ei 0000000000000000 T _ZN5ThingC2Ev U __gxx_personality_v0 </code></pre> <p>As you can see...the same function is generating multiple symbols, which is still quite curious.</p> <p>And while we're at it, here is a section of generated assembly:</p> <pre><code>.globl _ZN5ThingC2Ev .type _ZN5ThingC2Ev, @function _ZN5ThingC2Ev: .LFB1: .cfi_startproc .cfi_personality 0x3,__gxx_personality_v0 pushq %rbp .cfi_def_cfa_offset 16 movq %rsp, %rbp .cfi_offset 6, -16 .cfi_def_cfa_register 6 movq %rdi, -8(%rbp) leave ret .cfi_endproc .LFE1: .size _ZN5ThingC2Ev, .-_ZN5ThingC2Ev .align 2 .globl _ZN5ThingC1Ev .type _ZN5ThingC1Ev, @function _ZN5ThingC1Ev: .LFB2: .cfi_startproc .cfi_personality 0x3,__gxx_personality_v0 pushq %rbp .cfi_def_cfa_offset 16 movq %rsp, %rbp .cfi_offset 6, -16 .cfi_def_cfa_register 6 movq %rdi, -8(%rbp) leave ret .cfi_endproc </code></pre> <p>So the generated code is...well...the same.</p> <hr> <p><strong>EDIT</strong>: To see what constructor actually gets called, I changed <code>Thing::foo()</code> to this:</p> <pre><code>void Thing::foo() { Thing t; } </code></pre> <p>The generated assembly is:</p> <pre><code>.globl _ZN5Thing3fooEv .type _ZN5Thing3fooEv, @function _ZN5Thing3fooEv: .LFB550: .cfi_startproc .cfi_personality 0x3,__gxx_personality_v0 pushq %rbp .cfi_def_cfa_offset 16 movq %rsp, %rbp .cfi_offset 6, -16 .cfi_def_cfa_register 6 subq $48, %rsp movq %rdi, -40(%rbp) leaq -32(%rbp), %rax movq %rax, %rdi call _ZN5ThingC1Ev leaq -32(%rbp), %rax movq %rax, %rdi call _ZN5ThingD1Ev leave ret .cfi_endproc </code></pre> <p>So it is invoking the complete object constructor.</p>
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    1. USTravis Gockel
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    1. COYou're obfuscating your problem with the `-C` flag to `nm`. If you leave it off, you'll see that the constructors that are emitted in fact have different symbols (which is the answer to your second question). I have no idea why two identical constructors are emitted with different symbol names, but I'm trying to read up on that now... more if I figure it out.
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      1. USCarl Norum
    2. COYour output looks roughly the same as what I get here - so the question, really, is "what's the difference between the mangled name with a `C1` in it versus that with a `C2` in it?", and I have no answer to that question. I'm surprised the documentation doesn't have more about it.... hrm.
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      1. USCarl Norum
    3. COIts interesting that the exact same behavior happens in two different compilers.
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      1. USTravis Gockel
 

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