C++11 中引入 std::ref
用于取某个变量的引用,这个引入是为了解决一些传参问题。
我们知道 C++ 中本来就有引用的存在,为何 C++11 中还要引入一个 std::ref
了?主要是考虑函数式编程(如 std::bind
)在使用时,是对参数直接拷贝,而不是引用。下面通过例子说明
示例1:
#include<iostream> #include<functional> void f(int& n1, int& n2, const int& n3) { std::cout << "In function: " << n1 << ' ' << n2 << ' ' << n3 << ' '; ++n1; // increments the copy of n1 stored in the function object ++n2; // increments the main()'s n2 // ++n3; // compile error } int main() { int n1 = 1, n2 = 2, n3 = 3; std::function<void()> bound_f = std::bind(f, n1, std::ref(n2), std::cref(n3)); n1 = 10; n2 = 11; n3 = 12; std::cout << "Before function: " << n1 << ' ' << n2 << ' ' << n3 << ' '; bound_f(); std::cout << "After function: " << n1 << ' ' << n2 << ' ' << n3 << ' '; }
root@ubuntu:~/c++# ./obj Before function: 10 11 12 In function: 1 11 12 After function: 10 12 12 --n2增加1 root@ubuntu:~/c++#
f(int& n1, int& n2, const int& n3) 参数都是引用
上述代码在执行 std::bind
后,在函数 f()
中 n1
的值仍然是 1,n2
和 n3
改成了修改的值,说明 std::bind
使用的是参数的拷贝而不是引用,因此必须显示利用 std::ref
来进行引用绑定。具体为什么 std::bind
不使用引用,可能确实有一些需求,使得 C++11 的设计者认为默认应该采用拷贝,如果使用者有需求,加上 std::ref
即可。
示例2:
#include<iostream> #include<string> #include<thread> void threadFunc(std::string &str, int a) { str = "change by threadFunc"; a = 13; } int main() { std::string str("main"); int a = 9; std::thread th(threadFunc, std::ref(str), a); th.join(); std::cout<<"str = " << str << std::endl; std::cout<<"a = " << a << std::endl; return 0; }
root@ubuntu:~/c++# ./obj str = change by threadFunc a = 9 root@ubuntu:~/c++#
#include<iostream> #include<string> #include<thread> void threadFunc(std::string &str, int a) { str = "change by threadFunc"; a = 13; } int main() { std::string str("main"); int a = 9; std::thread th(threadFunc, std::ref(str), std::ref(a)); th.join(); std::cout<<"str = " << str << std::endl; std::cout<<"a = " << a << std::endl; return 0; }
root@ubuntu:~/c++# ./obj str = change by threadFunc a = 9 root@ubuntu:~/c++#
可以看到,和 std::bind
类似,多线程的 std::thread
也是必须显式通过 std::ref
来绑定引用进行传参,否则,形参的引用声明是无效的。
#include<iostream> #include<string> #include<thread> void threadFunc(std::string &str, int & a) { str = "change by threadFunc"; a = 13; } int main() { std::string str("main"); int a = 9; std::thread th(threadFunc, std::ref(str), std::ref(a)); th.join(); std::cout<<"str = " << str << std::endl; std::cout<<"a = " << a << std::endl; return 0; }
threadFunc(std::string &str, int & a) 都采用引用
root@ubuntu:~/c++# g++ -std=c++11 -pthread obj.cpp -o obj root@ubuntu:~/c++# ./obj str = change by threadFunc a = 13