[cfe-dev] Why a few std::string function definitions are not in the object code?

Douglas Gregor dgregor at apple.com
Fri Feb 18 14:01:04 PST 2011

On Feb 18, 2011, at 1:18 PM, Xiaolong Tang wrote:

> Hi all, 
> I am working on whole-program analysis, and expect the final bitcode
> to have all function definitions. When I examined the final bitcode of
> a simple program using the standard string, I found that some function
> defintions are missing. Particularly I am interested in the following
> function definitions which are missing in the final bitcode. 
>  // string constructor
>  declare void @_ZNSsC1Ev(%"struct.std::string")
>  // After name demangling 
>  declare void @std::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string()(%"struct.std::string")
>  // allocator constructor
>  declare void @_ZNSaIcEC1Ev(%"struct.std::allocator<char>") nounwind
>  // After name demangling 
>  declare void @std::allocator<char>::allocator()(%"struct.std::allocator<char>") nounwind
>  // string constructor from a C string
>  declare void @_ZNSsC1EPKcRKSaIcE(%"struct.std::string", i8, %"struct.std::allocator<char>")
>  // After name demangling 
>  declare void @std::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(char const*, std::allocator<char> const&)(%"struct.std::string", i8, %"struct.std::allocator<char>")
>  // string assignment
>  declare %"struct.std::string" @_ZNSsaSERKSs(%"struct.std::string", %"struct.std::string")
>  // After name demangling 
>  declare %"struct.std::string" @std::basic_string<char, std::char_traits<char>, std::allocator<char> >::operator=(std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)(%"struct.std::string", %"struct.std::string")
> So, I am wondering why it is so? Specifically, I have a few questions. 
> 1. Is the above compilation result related to the compilation mode and
> instantiation mechanism of C++ templates?

Yes. Your standard library is using extern templates to suppress the generation of these definitions in each translation unit. If you look in your standard library headers, you'll see something like:

	extern template class basic_string<char>;

> 2. Does clang (or linker) provide any way to force these function
>   definitions to be present in the final bitcode? 

If you can comment out or #define away those extern templates, you'll see all of the function definitions. I don't know of any easier way.

	- Doug

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