There is a problem with usage of typenames with struct in the old C++ standard. I have the following error:
error: no matching function for call to
This is what I get when compiling with C++98 standard. With C++11 and newer, it works as I expect.
I defined a struct with a template and in the main() function I initialized the Pair from my templated struct with some values. Then I print the members of the struct:
#include <iostream>
template <typename T>
struct Pair
{
T first = 0;
T second = 0;
};
int main()
{
Pair<int> p1 { 5, 6 };
std::cout << p1.first << ' ' << p1.second << '\n';
p1 = { 2, 3 };
std::cout << p1.first << ' ' << p1.second << '\n';
return 0;
}
GDB Compiler tells me:
main.cpp: In function ‘int main()’: main.cpp:12:25: error: no matching function for call to ‘Pair::Pair()’ 12 | Pair<int> p1 { 5, 6 };
But if we change declaration of members in the struct to this:
struct Pair
{
T first;
T second;
};
It works correctly.
Can somebody explain why it works like this?
>Solution :
Default member initializers were a feature added to the language in C++11. That means that the = 0 part of your Pair member declarations is ill-formed before C++11.
Uniform initialization syntax was also added in C++11, which means that Pair<int> p1 { 5, 6 } and p1 = { 2, 3 } are also ill-formed prior to C++11.
Changing the definition of Pair as you did fixes the first issue, but not the second. To get the same functionality with C++98 you will need to add user-defined constructors to Pair:
template <typename T>
struct Pair
{
T first;
T second;
Pair() : first(), second() {}
Pair(T f, T s) : first(f), second(s) {}
};
int main()
{
Pair<int> p1(5, 6);
std::cout << p1.first << ' ' << p1.second << '\n';
p1 = Pair<int>(2, 3);
std::cout << p1.first << ' ' << p1.second << '\n';
return 0;
}