The below example looks strange and pointless, but it produces what I believe to be the same error that I'm actually experiencing elsewhere.
template <typename T, typename F>
bool foo (const T & x, const F & a, const F & b)
{
return a (x) == b (x);
}
template <typename T>
bool bar (const T & x, const T & a, const T & b)
{
return foo (
x,
[&] (const T & arg) {return a == arg;},
[&] (const T & arg) {return b == arg;});
}
int main ()
{
bar (0, 0, 0);
}
The error is
no matching function for call to ‘foo(const int&, bar(const T&, const T&, const T&) [with T = int]::<lambda(const int&)>, bar(const T&, const T&, const T&) [with T = int]::<lambda(const int&)>)’
deduced conflicting types for parameter ‘const F’ (‘bar(const T&, const T&, const T&) [with T = int]::<lambda(const int&)>’ and ‘bar(const T&, const T&, const T&) [with T = int]::<lambda(const int&)>’)
If I put the lambdas into std::function that fixes the problem
std::function<bool(int)> f_a = [&] (const T & arg) {return a == arg;};
std::function<bool(int)> f_b = [&] (const T & arg) {return b == arg;};
return foo (x, f_a, f_b);
I can also fix it like this.
template <typename T, typename F1, typename F2>
bool foo (const T & x, const F1 & a, const F2 & b)
Based on this fix working, I assume it failed initially because the two compile could not determine that the arguments F&a and F&b actually have the same type. But they do have the same type.
I'm confused. Why doesn't the first version work?
