Re: A C2664 error

From:

Ondra Holub <ondra.holub@post.cz>

Newsgroups:

comp.lang.c++

Date:

Fri, 21 Dec 2007 00:23:54 -0800 (PST)

Message-ID:

<688a18e2-bf41-4e5c-91a4-3b6b779cab2f@l32g2000hse.googlegroups.com>

zhx...@gmail.com napsal:

Here is a program, I want to change the way which a map use to order
elements.

struct sizemore
{
    bool operator()(const wstring &s1,const wstring &s2)
    {
        return s1.size()>s2.size();
    }
};

int dictinput(const char *filename,map<wstring,wstring> &mwdict())
{
    ...
}

int main()
{
    map<wstring,wstring> mwdict(sizemore());
    dictinput("filename",mwdict);
}

But I receive a message:
error C2664: 'dictinput' : cannot convert parameter 2 from
'std::map<_Kty,_Ty> (__cdecl *)(sizemore (__cdecl *)(void))' to
'std::map<_Kty,_Ty> &(__cdecl *)(void)'

I'm always puzzled with parameters of function.
Could some one give me some advices?
Thanks a lot.

You need to specify comparation as 3rd parameter of template std::map.
See example below:

#include <iostream>

#include <map>
#include <string>

template<typename T>
struct CmpBySize
{
    bool operator()(const T& s1, const T& s2) const
    {
        return s1.size() < s2.size();
    }
};

typedef
    std::map<std::string, std::string, CmpBySize<std::string> >
MapStrStr;

int main()
{
    MapStrStr m;
    m["aaa"] = "3 x a";
    m["bb"] = "2 x b";
    m["z"] = "1 x z";

    const MapStrStr::const_iterator end = m.end();
    for (MapStrStr::const_iterator i = m.begin(); i != end; ++i)
    {
        std::cout << "m[\"" << i->first << "\"] = \"" << i->second <<
"\"\n";
    }
}