Re: returning std::string by value across DLL boundaries

Maxim Yegorushkin <>
Sun, 7 Jun 2009 23:16:06 CST
On 5 June, 18:54, ghita <> wrote:

Hi !

I want to be able to return from a function inside a DLL a std::string
by value:
1) std::string func();

In another module (DLL or exe) I want to use this even if I use
different c runtime linkages (example: func() inside the DLL uses
dynamic c runtime and the other module uses the same c runtime but

The way I have done successfully this when passing std::strings as
references inside a DLL was to make sure I do not resize by any means
the capacity of the string inside the DLL( so as to not delete
afterwards the string when memory was allocated inside another module,
the DLL)

Case 1 seems to work fine on my complier (Visual Studio) although I
don't really understand why ...

2) Is there any other better alternative because I don't want to stick
on using raw pointers when crossing DLL boundaries ?

The problem is caused by the mismatch of allocation and deallocation
functions (new/delete, malloc/free) from different C++ run-time. To
solve it, you need to make sure that objects are allocated and
deallocated by functions from the same C++ run-time. The easiest way
to do so it to allocate and deallocate objects in the same source

Applying this to std::string and std containers, what you need to do
is to use a custom allocator. That allocator will need to be in one
dll, so that allocator::allocate() and allocator::deallocate() use the
same C++ run-time.


// these two functions are implemented in one dll
__declspec(dllexport) void* dll_malloc(size_t);
__declspec(dllexport) void dll_free(void*);

template<class T>
struct dll_allocator
     // copy-intellegent-paste std::allocator implementation here

     // the only change is in the following two functions

     pointer allocate(size_type n, const void* = 0)
         if(void* p = dll_malloc(sizeof(T) * n))
             return p;
         throw std::bad_alloc();

     void deallocate(pointer p, size_type)

// now use dll_string between dll's boundaries
typedef std::basic_string<
     , std::char_traits<char>
     , dll_allocator<char>
     > dll_string;

// an example of dll_string_vector
typedef std::vector<
     , dll_allocator<dll_string>
     > dll_string_vector;


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