c++ - std::pair<> 的默认构造函数是否将基本类型(int 等)设置为零？

Question

写完后:

std::pair<int, int> x;

我能保证 x.first 和 x.second 都为零吗？或者它们有什么值(value)吗？

我关心的原因是因为我试图确定如果我访问不在 map 中的元素，是否保证值是指针的 map 返回 NULL。即，如果我这样做:

std::map<int, void*> my_map;
std::cout << int(my_map[5]) << std::endl;

那么我能保证得到零(NULL)吗？还是行为未定义？

Answer 1

是的，该保证成立。引用 C++11 标准，§20.3.2/2-3:

        constexpr pair();

2 Requires: is_default_constructible<first_type>::value is true and is_default_constructible<second_type>::value is true.
3 Effects: Value-initializes first and second.

以及§8.5/7:

To value-initialize an object of type T means:

• if T is a (possibly cv-qualified) class type with a user-provided constructor, then the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);
• if T is a (possibly cv-qualified) non-union class type without a user-provided constructor, then the object is zero-initialized and, if T's implicitly-declared default constructor is non-trivial, that constructor is called.
• if T is an array type, then each element is value-initialized;
• otherwise, the object is zero-initialized.

最后，§8.5/5:

To zero-initialize an object or reference of type T means:

• if T is a scalar type, the object is set to the value 0 (zero), taken as an integral constant expression, converted to T;
• if T is a (possibly cv-qualified) non-union class type, each non-static data member and each base-class subobject is zero-initialized and padding is initialized to zero bits;
• if T is a (possibly cv-qualified) union type, the object’s first non-static named data member is zero-initialized and padding is initialized to zero bits;
• if T is an array type, each element is zero-initialized;
• if T is a reference type, no initialization is performed.