C++11学习：auto和decltype_c++墨汁指针-优快云博客

本文链接：https://blog.youkuaiyun.com/cxxmaker/article/details/9622541

本文通过一个加法函数的多种实现方式，展示了从C++03到C++11的不同风格，并重点介绍了C++11中自动类型推导、通用返回类型等新特性。同时，通过对不同方式的对比，帮助读者理解这些新特性如何简化模板函数的定义。

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#include <iostream>

/* C++03 style */
template <typename R, typename T1, typename T2>
R add(T1 t1, T2 t2)
{
    std::cout << "C++03 style" << std::endl;
    return(t1 + t2);
}

/* C++11 style */

/* way 1 */
template <typename T1, typename T2>
auto add(T1 t1, T2 t2) ->decltype(t1 + t2)
{
    std::cout << "C++11 style, way 1" << std::endl;
    return(t1 + t2);
}

/* way 2 */
/*
template <typename T1, typename T2>
decltype(*(T1*)0 * *(T2*)0) add(T1 t1, T2 t2)
{
    std::cout << "C++11 style, way 2" << std::endl;
    return(t1 + t2);
}
*/

/* way 3*/
/*
template <typename T1, typename T2>
decltype(std::declval<T1>() * std::declval<T2>()) add(T1 t1, T2 t2)
{
    std::cout << "C++11 style, way 3" << std::endl;
    return(t1 + t2);
}
*/

/* way 4 */
/*
template <typename T>
struct DeclVal
{
    T operator () ();
};

template <typename T1, typename T2>
decltype(DeclVal<T1>()() * DeclVal<T2>()()) add(T1 t1, T2 t2)
{
    std::cout << "C++11 style, way 4" << std::endl;
    return(t1 + t2);
}
*/

/* way 5 */
/*
template <typename T>
T decl_val();

template <typename T1, typename T2>
decltype(decl_val<T1>() * decl_val<T2>()) add(T1 t1, T2 t2)
{
    std::cout << "C++11 style, way 5" << std::endl;
    return(t1 + t2);
}
*/

int main(int argc, char * argv[])
{
    auto d = 0.0;
    auto i = sizeof(d);
    std::cout << "sizeof(0.0) = " << i << std::endl;

    auto v1 = 5;
    auto v2 = 3.8;
    auto v3 = add(v1, v2);
    decltype(v1 + v2) v4 = add(v1, v2);
    auto v5 = add<double>(v1, v2);
    std::cout << "v1 = " << v1 << std::endl;
    std::cout << "v2 = " << v2 << std::endl;
    std::cout << "v3 = " << v3 << std::endl;
    std::cout << "v4 = " << v4 << std::endl;
    std::cout << "v5 = " << v5 << std::endl;

    return(0);
}