How do I avoid implicit conversions on non-constructing functions?

Define function template which matches all other types:

void function(int); // this will be selected for int only

template <class T>
void function(T) = delete; // C++11 

This is because non-template functions with direct matching are always considered first. Then the function template with direct match are considered – so never function<int> will be used. But for anything else, like char, function<char> will be used – and this gives your compilation errrors:

void function(int) {}

template <class T>
void function(T) = delete; // C++11 


int main() {
   function(1);
   function(char(1)); // line 12
} 

ERRORS:

prog.cpp: In function 'int main()':
prog.cpp:4:6: error: deleted function 'void function(T) [with T = char]'
prog.cpp:12:20: error: used here

This is C++03 way:

// because this ugly code will give you compilation error for all other types
class DeleteOverload
{
private:
    DeleteOverload(void*);
};


template <class T>
void function(T a, DeleteOverload = 0);

void function(int a)
{}