c++ - RVO：即使显式分配给 const 引用，也按值传递返回值

Question

我有一个设置框架，它最终将值缓存到一个std::mapof 中boost::any。

由于我不希望客户端处理异常，它提供了一个默认值，设置框架将在设置检索失败的情况下回退：这迫使我通过复制返回设置值。

class SettingsMgr
{

    public:
        template<class T>
        T getSetting(const std::string& settingName, const T& settingDefValue)
        {
            try
            {
                if(cache.find(settingName) != cache.end)
                {
                     return any_cast<const T&>(cache.find(settingName)->second);
                }
                else
                {
                    cache[settingName] = someDbRetrievalFunction<T>(settingName);    
                    return any_cast<const T&>(cache.find(settingName)->second);
                }
             }
             catch(...)
             {
                 return settingDefValue;
             }
          }
        // This won't work in case the default value needs to be returned 
        // because it would be a reference to a value the client - and not the SettingsMgr - 
        // owns (which might be temporary etc etc)
        template<class T>
        const T& getSettingByRef(const std::string& settingName, const T& settingDefValue);

     private:
        std::map<std::string, boost::any> cache;
}

现在，我并不认为这有什么大不了的，因为我认为多亏了 RVO 魔术，对设置框架拥有的缓存值的引用会被返回——尤其是当客户端显式地将返回值封装在 const 引用中时！

根据我的测试，情况似乎并非如此。

void main() {
    SettingsMgr sm;
    // Assuming everything goes fine, SNAME is cached
    const std::string& asettingvalue1 = sm.getSetting<std::string>("SNAME", "DEF_VALUE"); 

    // Assuming everything goes fine, cached version is returned (no DB lookup)
    const std::string& asettingvalue2 = sm.getSetting<std::string>("SNAME", "DEF_VALUE");

    ASSERT_TRUE(&asettingvalue1 == &asettingvalue2);  // Fails

    const std::string& awrongsettingname = sm.getSettingByRef<std::string>("WRONGSETTINGNAME", "DEF_VALUE");
    ASSERT_TRUE(awrongsettingname  == "DEF_VALUE"); // Fails, awrongsettingname is random memory
}

Answer 1

您可以使用该getSettingByRef版本并防止传递任何右值引用的可能性：

    template<class T>
    const T & getSetting(const std::string& settingName, T&& settingDefValue) {
         static_assert(false, "No rvalue references allowed!");
    }