严格规则什么构成复制构造函数（cf. C ++ 11，12.8）：

• 它不是模板。 / p>




• 对于类 T ，其第一个参数必须具有 或 T const& 或 T volatile&




• 如果它有多个参数，进一步的参数必须有默认值。

如果不声明复制构造函数，则 T :: T T const&）是隐式声明。 （它可能或可能不会被实际定义，如果它被定义，它可能被定义为已删除。）

（通常的重载解析规则意味着你可以）

对于move构造函数有类似的规则，&& 代替& 。

I read that template copy-con is never default copy onstructor, and template assignment-op is never a copy assignment operator.

I couldn't understand why this restriction is needed and straight away went online to ideone and return a test program but here copy constructor never gets called on further googling I came across templatized constructor and tried that but still it never calls copy constructor.

#include <iostream>
using namespace std;

template <typename T> class tt
{
    public :
    tt()
    {
        std::cout << std::endl << "   CONSTRUCTOR" << std::endl;
    }
    template <typename U> const tt<T>& operator=(const tt<U>& that){std::cout << std::endl << "   OPERATOR" << std::endl;}
    template <typename U> tt(const tt<U>& that)
    {
        std::cout << std::endl << "    COPY CONSTRUCTOR" << std::endl;
    }
};


tt<int> test(void)
{
    std::cout << std::endl << "      INSIDE " << std::endl; tt<int> a; return a;
}

int main() {
    // your code goes here
    tt<int> a ; a = test();

    return 0;
}

Can someone explain me the whole reason behind putting this restriction and also how to write a copy constructor of template class.

Thanks

There are strict rules what constitutes a copy constructor (cf. C++11, 12.8):

• It is not a template.

• For a class T, its first argument must have type T & or T const & or T volatile & or T const volatile &.

• If it has more than one argument, the further arguments must have default values.

If you do not declare a copy constructor, a copy constructor of the form T::T(T const &) is implicitly declared for you. (It may or may not actually be defined, and if it is defined it may be defined as deleted.)

(The usual overload resolution rules imply that you can have at most four copy constructors, one for each CV-qualification.)

There are analogous rules for move constructors, with && in place of &.