现代C++技术研究（4）---引用折叠规则

C++11对类型推导遵守所谓引用折叠规则（reference collapsing）。举个例子：

#include 
#include 

int main()
{
    using T = const int;
    using TR = T&;
    // case 1
    using ResultType = TR;
    bool isCnstInt = std::is_same::value;
    bool isCnstIntRef = std::is_same::value;
    bool isCnstIntRightRef = std::is_same::value;
    if (isCnstInt) {
        std::cout << "Case1:ResultType is const int\n";
    } else if (isCnstIntRef) {
        std::cout << "Case1:ResultType is const int&\n";
    } else if (isCnstIntRightRef) {
        std::cout << "Case1:ResultType is const int&&\n";
    }
    // case 2
    using ResultType2 = TR&;
    bool isCnstInt2 = std::is_same::value;
    bool isCnstIntRef2 = std::is_same::value;
    bool isCnstIntRightRef2 = std::is_same::value;
    if (isCnstInt2) {
        std::cout << "Case2:ResultType is const int\n";
    } else if (isCnstIntRef2) {
        std::cout << "Case2:ResultType is const int&\n";
    } else if (isCnstIntRightRef2) {
        std::cout << "Case2:ResultType is const int&&\n";
    }
    // case 3
    using ResultType3 = TR&&;
    bool isCnstInt3 = std::is_same::value;
    bool isCnstIntRef3 = std::is_same::value;
    bool isCnstIntRightRef3 = std::is_same::value;
    if (isCnstInt3) {
        std::cout << "Case3:ResultType is const int\n";
    } else if (isCnstIntRef3) {
        std::cout << "Case3:ResultType is const int&\n";
    } else if (isCnstIntRightRef3) {
        std::cout << "Case3:ResultType is const int&&\n";
    }
    using TR2 = T&&;
    // case 4
    using ResultType4 = TR2;
    bool isCnstInt4 = std::is_same::value;
    bool isCnstIntRef4 = std::is_same::value;
    bool isCnstIntRightRef4 = std::is_same::value;
    if (isCnstInt4) {
        std::cout << "Case4:ResultType is const int\n";
    } else if (isCnstIntRef4) {
        std::cout << "Case4:ResultType is const int&\n";
    } else if (isCnstIntRightRef4) {
        std::cout << "Case4:ResultType is const int&&\n";
    }
    // case 5
    using ResultType5 = TR2&;
    bool isCnstInt5 = std::is_same::value;
    bool isCnstIntRef5 = std::is_same::value;
    bool isCnstIntRightRef5 = std::is_same::value;
    if (isCnstInt5) {
        std::cout << "Case5:ResultType is const int\n";
    } else if (isCnstIntRef5) {
        std::cout << "Case5:ResultType is const int&\n";
    } else if (isCnstIntRightRef5) {
        std::cout << "Case5:ResultType is const int&&\n";
    }
    // case 6
    using ResultType6 = TR2&&;
    bool isCnstInt6 = std::is_same::value;
    bool isCnstIntRef6 = std::is_same::value;
    bool isCnstIntRightRef6 = std::is_same::value;
    if (isCnstInt6) {
        std::cout << "Case6:ResultType is const int\n";
    } else if (isCnstIntRef6) {
        std::cout << "Case6:ResultType is const int&\n";
    } else if (isCnstIntRightRef6) {
        std::cout << "Case6:ResultType is const int&&\n";
    }
    return 0;
}

这个小程序的测试结果是：

[root@192 moderncpp]# ./reference_collapse
Case1:ResultType is const int&
Case2:ResultType is const int&
Case3:ResultType is const int&
Case4:ResultType is const int&&
Case5:ResultType is const int&
Case6:ResultType is const int&&

我们可以把测试结果整理成如下表格：

C++11中的引用折叠规则

TR的类型定义	ResultType的类型定义	ResultType的类型推导结果
T&	TR	T&
T&	TR&	T&
T&	TR&&	T&
T&&	TR	T&&
T&&	TR&	T&
T&&	TR&&	T&&

可以看出，对于TR的类型定义或者ResultType的类型定义，只要出现左值引用，那么类型推导结果一定是左值引用，在没有左值引用的情况下，如果出现右值引用，那么类型推导结果会是右值引用。

参考资料：

《深入理解C++11：C++11新特性解析与应用》