好记性不如烂笔头，内容来自cppreference.com.
语法 🔗
c++11 的 lambda capture 大概有如下语法：
identifier |
(1) | |
|---|---|---|
identifier ... |
(2) | |
identifier initializer |
(3) | (since C++14) |
& identifier |
(4) | |
& identifier ... |
(5) | |
& identifier initializer |
(6) | (since C++14) |
this |
(7) | |
* this |
(8) | (since C++17) |
... identifier initializer |
(9) | (since C++20) |
& ... identifier initializer |
(10) | (since C++20) |
其中，
- (1) simple by-copy capture
- (2) simple by-copy capture that is a pack expansion
- (3) by-copy capture with an initializer
- (4) simple by-reference capture
- (5) simple by-reference capture that is a pack expansion
- (6) by-reference capture with an initializer
- (7) simple by-reference capture of the current object
- (8) simple by-copy capture of the current object
- (9) by-copy capture with an initializer that is a pack expansion
- (10) by-reference capture with an initializer that is a pack expansion
如果某个 lambda 的 capture-default 是 &, 则后续的 simple capture 不能再以 & 打头 – 没啥意义。
示例1 🔗
struct S2 { void f(int i); };
void S2::f(int i)
{
[&] {}; // OK: by-reference capture default
[&, i] {}; // OK: by-reference capture, except i is captured by copy
[&, &i] {}; // Error: by-reference capture when by-reference is the default
[&, this] {}; // OK, equivalent to [&]
[&, this, i] {}; // OK, equivalent to [&, i]
}
同理， 如果某个 lambda 的 capture-default 是 =, 则后续的 simple capture 必须以 & 打头。
示例2 🔗
struct S2 { void f(int i); };
void S2::f(int i)
{
[=] {}; // OK: by-copy capture default
[=, &i] {}; // OK: by-copy capture, except i is captured by reference
[=, *this] {}; // until C++17: Error: invalid syntax
// since C++17: OK: captures the enclosing S2 by copy
[=, this] {}; // until C++20: Error: this when = is the default
// since C++20: OK, same as [=]
}
示例3 🔗
capture 只能出现一次，且不能和参数重名。
struct S2 { void f(int i); };
void S2::f(int i)
{
[i, i] {}; // Error: i repeated
[this, *this] {}; // Error: "this" repeated (C++17)
[i] (int i) {}; // Error: parameter and capture have the same name
}