yonutix's questions

是否可以有一个带有可选模板参数的类，可以像这样调用？：

#include <iostream>

template <typename T = void>
class A final
{
public:
    // This class can be called only when T exists.
    void f()
    {
        printf("%d\n", member);
    }

    // This method can be called only when T is missing.
    void g()
    {
        printf("No template parameter\n");
    }
public:
    T member;
};

int main()
{

    A<int> a1;
    A a2;
    a1.f(); // should be valid
    a1.g(); // should be invalid, cannot compile
    a2.f(); // should be invalid, cannot compile
    a2.g(); // should be valid

    return 0;
}

如果是，应该使用哪些 std 函数？