例：
std::unordered_map<void *,size_t> size; char *test = new char[10]; size[test] += 10;
在此序列结束时,尺寸[测试]是否保证为10？
解决方法
Is size[test] guaranteed to be 10 at the end of this sequence?
是.
在代码的最后一行,size [test] value-将元素初始化为T(),或者在本例中为size_t()：
C++11 23.4.4.3 map element access [map.access]
T& operator[](const key_type& x);1 Effects: If there is no key equivalent to x in the map,inserts value_type(x,T()) into the map.
对于T(),确切的语言有点涉及,所以我将尝试引用相关的位：
C++11 8.5.16 The semantics of initializers are as follows.
— If the initializer is (),the object is value-initialized.
8.5.7 To value-initialize an object of type T means:
— if T is a (possibly cv-qualified) class type …
— if T is a (possibly cv-qualified) non-union class type …
— if T is an array type,then each element is value-initialized;
— otherwise,the object is zero-initialized.
8.5.5 To zero-initialize an object or reference of type T means:
— if T is a scalar type (3.9),the object is set to the value 0 (zero),taken as an integral constant expression,converted to T;
