Question

I’m frequently troubled by const correctness, and this seems to be no exception. Please advise me why the following code wont compile:

class string_token_stream
{
    public:

    typedef wchar_t* string_type;

    string_token_stream(const string_type input_string)
        : _input_string(input_string)
    {
    }

    private:

    const string_type _input_string;
};

int main(int argc, char **argv)
{
    const wchar_t *str = get_a_string_somewhere();
    string_token_stream sts(str);
    //                      ^------ Compile-time error C2664.
    return 0;
}

The error given is:

error C2664: 'string_token_stream::string_token_stream(const string_token_stream::string_type)' : cannot convert parameter 1 from 'const wchar_t *' to 'const string_token_stream::string_type'
1>          Conversion loses qualifiers

I’m compiling on Visual C++ 2010 Express. Additional compile/linker options available upon request.

Answer 1

When you write const string_type input_string the const is ignored for typedef. So you are trying to convert const wchar_t* to a wchar_t*.
If you changetypedef wchar_t* string_type; to typedef const wchar_t* string_type; it should compile.

const string_type input_string would look like wchar_t *const input_string whereas you expected const wchar_t*. You can see this by changing const wchar_t *str to wchar_t *const str and it will also work.