g++ compiler error: couldn't deduce template parameter ‘_Funct’

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);

Answer 2

std::for_each declaration looks like this:

template<class InputIter, class Func>
void for_each(InputIter first, InputIter last, Func func);

As you can see, it takes anything you give it as the third parameter. There is no restriction that it has to be a callable type of a certain signature or a callable type at all.

When dealing with overloaded functions, they're inherently ambiguous unless you give them some context to select the right one. In a call to an overloaded function, this context are the arguments you pass. When you need a pointer, however, you can't use arguments as a context, and the for_each parameter also doesn't count as a context, since it takes anything.

As an example of where a function parameter can be a valid context to select the right overload, see this:

// our overloads
void f(int){}
void f(double){}

typedef void (*funcptr_type)(int);
void g(funcptr_type){}

// ...
g(&f); // will select 'void f(int)' overload, since that's 
       // the only valid one given 'g's parameter

As you can see, you give a clear context here that helps the compiler select the right overload and not have it ambiguous. std::for_each's parameters do not give such a context, since they take anything.

There are two solutions:

• manually provide the context either by
  • casting to the right function pointer type, or
  • using an intermediate variable of the right type and passing that
• use a non-overloaded function that dispatches to an overloaded one (as you did with f)

Note that in C++11, you could also use a lambda for the second option:

std::for_each(v.begin(), v.end(), [](const S& s){ print_struct(s); });

---

Some notes on your code:

• (struct S){'a', 1} is a compound literal and not standard C++
• you don't need struct S in C++, only S suffices