使用花括号初始值设定项的默认参数
Default argument using curly braces initializer
我有这段代码似乎运行良好：
class foo{/* some member variables and functions*/};
void do_somthing(foo x={}){}
int main(){
do_somthing();
}
我曾经使用 void do_somthing(foo x=foo()){} 来默认 x 参数，但我在某些书籍或在线示例中看到这种方式 ={}（不记得了）。使用它完全没问题吗？这两种方法有什么区别吗？
foo x=foo() 是 copy initialization、
Initializes an object from another object
和 foo() 是 value initialization。
This is the initialization performed when a variable is constructed with an empty initializer.
foo x={} 是 aggregate initialization.
Initializes an aggregate from braced-init-list
If the number of initializer clauses is less than the number of
members and bases (since C++17) or initializer list is completely
empty, the remaining members and bases (since C++17) are initialized
by their default initializers, if provided in the class definition,
and otherwise (since C++14) by empty lists, which performs
value-initialization.
所以在这种情况下结果是一样的（都是值初始化的）。
而本例中值初始化的效果是：
if T is a class type with a default constructor that is neither user-provided nor deleted (that is, it may be a class with an implicitly-defined or defaulted default constructor), the object is zero-initialized
最终零初始化在这种情况下的效果是：
If T is a scalar type, the object's initial value is the integral
constant zero explicitly converted to T.
If T is an non-union class type, all base classes and non-static data
members are zero-initialized, and all padding is initialized to zero
bits. The constructors, if any, are ignored.
我有这段代码似乎运行良好：
class foo{/* some member variables and functions*/};
void do_somthing(foo x={}){}
int main(){
do_somthing();
}
我曾经使用 void do_somthing(foo x=foo()){} 来默认 x 参数，但我在某些书籍或在线示例中看到这种方式 ={}（不记得了）。使用它完全没问题吗？这两种方法有什么区别吗？
foo x=foo() 是 copy initialization、
Initializes an object from another object
和 foo() 是 value initialization。
This is the initialization performed when a variable is constructed with an empty initializer.
foo x={} 是 aggregate initialization.
Initializes an aggregate from braced-init-list
If the number of initializer clauses is less than the number of members and bases (since C++17) or initializer list is completely empty, the remaining members and bases (since C++17) are initialized by their default initializers, if provided in the class definition, and otherwise (since C++14) by empty lists, which performs value-initialization.
所以在这种情况下结果是一样的（都是值初始化的）。
而本例中值初始化的效果是：
if T is a class type with a default constructor that is neither user-provided nor deleted (that is, it may be a class with an implicitly-defined or defaulted default constructor), the object is zero-initialized
最终零初始化在这种情况下的效果是：
If T is a scalar type, the object's initial value is the integral constant zero explicitly converted to T.
If T is an non-union class type, all base classes and non-static data members are zero-initialized, and all padding is initialized to zero bits. The constructors, if any, are ignored.