C++ template compile failed [duplicate] - c++

Question

This question already has answers here:
Template class with template container
(3 answers)
Closed 6 months ago.
I want write an simple demo, but it compile failed, I can't find the reason.
#include <iostream>
#include <vector>
#include <list>
using namespace std;
template <
typename T,
template <typename W> typename Container = std::vector
>
class myclass
{
public:
Container<T> myc;
public:
void func();
myclass()
{
for (int i = 0; i < 10; ++i)
{
myc.push_back(i);
}
}
};
template <
typename T,
template <typename W> typename Container
>
void myclass<T, Container>::func()
{
cout << "good!" << endl;
}
int main()
{
myclass<int, vector> mylistobj2;
mylistobj2.func();
return 0;
}
the compile reason is that
tmp.cpp: In function ‘int main()’:
tmp.cpp:100:30: error: type/value mismatch at argument 2 in template parameter list for ‘template<class T, template<class W> class Container> class _nmsp1::myclass’
100 | _nmsp1::myclass<double, list> mylistobj2;
| ^
tmp.cpp:100:30: note: expected a template of type ‘template<class W> class Container’, got ‘template<class _Tp, class _Alloc> class std::__cxx11::list’
tmp.cpp:101:13: error: request for member ‘func’ in ‘mylistobj2’, which is of non-class type ‘int’
101 | mylistobj2.func();

Answer 1

std::vector has two template parameters (the 2nd one has default argument), while the template template parameter Container is declared with only one template parameter. They don't match. You may apply parameter pack.
template <
typename T,
template <typename...> typename Container = std::vector
>
class myclass
{
public:
Container<T> myc;
public:
void func();
myclass()
{
for (int i = 0; i < 10; ++i)
{
myc.push_back(i);
}
}
};
template <
typename T,
template <typename...> typename Container
>
void myclass<T, Container>::func()
{
cout << "good!" << endl;
}
Your code would work fine with C++17; since C++17 (CWG 150), the default template arguments are allowed for a template template argument to match a template template parameter with fewer template parameters.

Answer 2

The problem is that prior to C++17
a template template argument had to be a template with parameters that exactly match the parameters of the template template parameter it substitutes with some exceptions related to variadic template parameters.
In your given example this means that since std::vector template of the standard library has more than one parameter: The second parameter (which describes an allocator) has a default value, but prior to C++17 this was not considered when matching std::vector to the Container parameter.
There are 2 ways to solve this.
Solution 1
One solution is to rewrite the class myclass declaration so that the Container parameter expects containers with two template parameters as shown below:
template <typename T,
template <typename W,typename Alloc = std::allocator<W>> typename Container = std::vector> //note the second template parameter Alloc added here
class myclass
{
public:
Container<T> myc;
public:
void func();
myclass()
{
for (int i = 0; i < 10; ++i)
{
myc.push_back(i);
}
}
};
template <typename T,
template <typename W, typename Alloc> typename Container> //note the second template parameter here
void myclass<T, Container>::func()
{
cout << "good!" << endl;
}
The output of the above modified program can be seen here.
Solution 2
You can make use of parameter pack to solve this problem.
template <typename T,
template <typename...> typename Container = std::vector> //note we have parameter pack
class myclass
{
public:
Container<T> myc;
public:
void func();
myclass()
{
for (int i = 0; i < 10; ++i)
{
myc.push_back(i);
}
}
};
template <typename T,
template <typename...> typename Container> //note we have parameter pack
void myclass<T, Container>::func()
{
cout << "good!" << endl;
}
The output of the above modified program can be seen here.
Note also that your program works without any modification with C++17.