我们可以使用 lambda 表达式作为函数参数的默认值吗?
参考 C++11 规范 (5.1.2.13):
Refering to the C++11 specification (5.1.2.13):
出现在默认参数中的 lambda 表达式不应隐式或显式捕获任何实体.
[ 示例:
A lambda-expression appearing in a default argument shall not implicitly or explicitly capture any entity.
[ Example:
void f2() {
int i = 1;
void g1(int = ([i]{ return i; })()); // ill-formed
void g2(int = ([i]{ return 0; })()); // ill-formed
void g3(int = ([=]{ return i; })()); // ill-formed
void g4(int = ([=]{ return 0; })()); // OK
void g5(int = ([]{ return sizeof i; })()); // OK
}
――结束示例 ]
但是,我们是否也可以使用 lambda 表达式本身作为函数参数的默认值?
However, can we also use a lambda-expression itself as the default value for a function argument?
例如
template<typename functor>
void foo(functor const& f = [](int x){ return x; })
{
}
推荐答案
是的.在这方面,lambda 表达式与其他表达式(例如 0
)没有区别.但请注意,默认参数不使用推导.换句话说,如果你声明
Yes. In this respect lambda expressions are no different from other expressions (like, say, 0
). But note that deduction is not used with defaulted parameters. In other words, if you declare
template<typename T>
void foo(T = 0);
then foo(0);
将调用 foo
但 foo()
是格式错误的.您需要显式调用 foo
.由于在您的情况下您使用的是 lambda 表达式,因此没有人可以调用 foo
,因为表达式的类型(在默认参数的位置)是唯一的.但是你可以这样做:
then foo(0);
will call foo<int>
but foo()
is ill-formed. You'd need to call foo<int>()
explicitly. Since in your case you're using a lambda expression nobody can call foo
since the type of the expression (at the site of the default parameter) is unique. However you can do:
// perhaps hide in a detail namespace or some such
auto default_parameter = [](int x) { return x; };
template<
typename Functor = decltype(default_parameter)
>
void foo(Functor f = default_parameter);
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