默认捕获

默认情况下，无法从 lambda 主体中访问未在捕获列表中明确指定的局部变量。但是，可以隐式捕获 lambda 体命名的变量：

int a = 1;
int b = 2;

// Default capture by value
[=]() { return a + b; }; // OK; a and b are captured by value

// Default capture by reference
[&]() { return a + b; }; // OK; a and b are captured by reference

显式捕获仍然可以与隐式默认捕获一起完成。显式捕获定义将覆盖默认捕获：

int a = 0;
int b = 1;

[=, &b]() {
    a = 2; // Illegal; 'a' is capture by value, and lambda is not 'mutable'
    b = 2; // OK; 'b' is captured by reference
};