Using std::bind in template function parameter - c++

Question

This has probably been asked in some way before, but I cannot find the right search keywords.
While writing testing functions, I decided to refactor the testing code into a template function:
#include <iostream>
#include <functional>
#include <utility>
#include <vector>
template <typename In, typename Exp >
void runTest (
std::pair<In, Exp> testParams,
Exp (*testFunction)(In)
/*std::function< Exp(In)> testFunction */ )
{
Exp result = testFunction(testParams.first);
std::cout << "Result : " << (result == testParams.second? "SUCCESS":"FAILURE")
<< " expected : " << testParams.second
<< " got : " << result
<< std::endl;
}
Fill a vector with input and expected results and pass the pairs along with the function we want to test. Worked great for one function:
long f1 (long a1)
{
return a1 + 100;
}
void testf1()
{
std::vector<std::pair<long, long> > testCases = {
{100,200},
{300,400}
};
for (auto test : testCases) {
runTest (test, f1);
}
}
but then had to test one that takes two parameters. "Ok, no problem, I'll std::bind1st... oh, that's deprecated... std::bind should to it though, right? the first argument and pass that along to runTest"...
long f2 (long a1, long a2)
{
return a1+a2;
}
void testf2()
{
long a1 = 1234;
std::vector<std::pair<long, long> > testCases = {
{0,1234},
{2,1238},
{11,1245}
};
for (auto test : testCases){
auto f2bound = std::bind(f2, a1, std::placeholders::_2);
runTest (test, f2bound);
}
}
But the compiler says 'no' :
~/src/cpplay/onestens$ g++ -m64 --std=c++11 -o soq.out soQuestionBind.cpp -g
soQuestionBind.cpp: In function ‘void testf2()’:
soQuestionBind.cpp:50:31: error: no matching function for call to ‘runTest(std::pair<long int, long int>&, std::_Bind<long int (*(long int, std::_Placeholder<2>))(long int, long int)>&)’
runTest (test, f2bound);
^
soQuestionBind.cpp:7:6: note: candidate: template<class In, class Exp> void runTest(std::pair<_T1, _T2>, Exp (*)(In))
void runTest (
^
soQuestionBind.cpp:7:6: note: template argument deduction/substitution failed:
soQuestionBind.cpp:50:31: note: mismatched types ‘Exp (*)(In)’ and ‘std::_Bind<long int (*(long int, std::_Placeholder<2>))(long int, long int)>’
runTest (test, f2bound);
^
I'm a bit behind C++11 (and 14 and 17) but this should be possible, right?
I guess the object returned by std::bind couldn't be coerced into a simple function pointer... so how must my template parameter be defined to accept the bound function?

Answer 1

I guess the object returned by std::bind couldn't be coerced into a simple function pointer.
Correct.
The typical way of getting generic callables is to use a simple template parameter. No need to use function pointers, std::function or whatever:
template <typename T, typename U, typename F>
void runTest (
std::pair<T, U> testParams,
F testFunction )
{
auto result = testFunction(testParams.first);
// do something ...
}
Now you can use std::bind, but I'd recommend to use a lambda instead.