Question

I’m trying to use an ANSI C++ for_each statement to iterate over and print the elements of a standard vector. It works if I have the for_each call a non-overloaded function, but yields a compiler error if I have it call an overloaded function.

Here’s a minimal test program to show where the compiler error occurs:

#include <algorithm>
#include <iostream>
#include <vector>

struct S {
    char c;
    int i;
};
std::vector<S> v;

void print_struct(int idx);
void print_struct(const struct S& s);

// f: a non-overloaded version of the preceding function.
void f(const struct S& s);

int main()
{
    v.push_back((struct S){'a', 1});
    v.push_back((struct S){'b', 2});
    v.push_back((struct S){'c', 3});

    for (unsigned int i = 0; i < v.size(); ++i)
        print_struct(i);

    /* ERROR! */
    std::for_each(v.begin(), v.end(), print_struct);

    /* WORKAROUND: */
    std::for_each(v.begin(), v.end(), f);

    return 0;
}

// print_struct: Print a struct by its index in vector v.
void print_struct(int idx)
{
    std::cout << v[idx].c << ',' << v[idx].i << '\n';
}

// print_struct: Print a struct by reference.
void print_struct(const struct S& s)
{
    std::cout << s.c << ',' << s.i << '\n';
}

// f: a non-overloaded version of the preceding function.
void f(const struct S& s)
{
    std::cout << s.c << ',' << s.i << '\n';
}

I compiled this in openSUSE 12.2 using:

g++-4.7 -ansi -Wall for_each.cpp -o for_each

The full error message is:

for_each.cpp: In function ‘int main()’:
for_each.cpp:31:48: error: no matching function for call to ‘for_each(std::vector<S>::iterator, std::vector<S>::iterator, <unresolved overloaded function type>)’
for_each.cpp:31:48: note: candidate is:
In file included from /usr/include/c++/4.7/algorithm:63:0,
                 from for_each.cpp:5:
/usr/include/c++/4.7/bits/stl_algo.h:4436:5: note: template<class _IIter, class _Funct> _Funct std::for_each(_IIter, _IIter, _Funct)
/usr/include/c++/4.7/bits/stl_algo.h:4436:5: note:   template argument deduction/substitution failed:
for_each.cpp:31:48: note:   couldn't deduce template parameter ‘_Funct’

I don’t see any search results for this particular error on Stack Overflow, or on the web generally. Any help would be appreciated.

Answer 1

A names refers to an overload set. You’ll need to specify which overload you want:

std::for_each(v.begin(), v.end(), (void (&)(S const&)) print_struct);

Another approach is to use a polymorphic callable function object as a helper:

struct PrintStruct
{
    template <typename T> void operator()(T const& v) const 
        { return print_struct(v); }
};

int main()
{
    PrintStruct helper;

    std::vector<S> sv;
    std::vector<int> iv;

    // helper works for both:
    std::for_each(sv.begin(), sv.end(), helper);
    std::for_each(iv.begin(), iv.end(), helper);