C++：error C2228: left of '.str' must have class/struct/union

逃离我推掉我的手 2022-05-15 13:58 186阅读 0赞

如下代码报error C2228: left of '.str' must have class/struct/union

#include <string>
 #include <iostream>
 #include <vector>
 
 using namespace std;
 
 template <typename T>
 class ValueBox {
 private:
 T value;
 
 private:
 
 template<typename U, 
 typename std::enable_if<std::is_class::value && !std::is_same<U, string>::value, U>::type* = nullptr, 
 typename std::enable_if<!class_str::ret, U>::type* = nullptr>
 std::string str(const T&) {
 cout << "1.---------------------" << endl;
 return "null";
 };
 
 template<typename U, 
 typename std::enable_if<std::is_class::value && std::is_same<U, string>::value, U>::type* = nullptr>
 std::string str(const T&) {
 cout << "2.---------------------" << endl;
 return value;
 };
 
 template<typename U, 
 typename std::enable_if<std::is_class::value && !std::is_same<U, string>::value, U>::type* = nullptr, 
 typename std::enable_if<class_str::ret, U>::type* = nullptr>
 std::string str(const T&) {
 cout << "3.---------------------" << endl;
 return value.str();
 };
 
 template<typename U, 
 typename std::enable_if<!std::is_class::value && std::is_arithmetic::value, U>::type* = nullptr>
 std::string str(const T&) {
 cout << "4.---------------------" << endl;
 return std::to_string(value);
 };
 
 public:
 ValueBox(const T& _value) : value(_value) {
 }
 
 const T& getValue() const {
 return value;
 };
 
 T& getValue() {
 return value;
 };
 
 std::string str() {
 return str<T>(value);
 };
 };
 
 
 int main() {
 ValueBox<string> s("sddds");
 cout << s.str() << endl;
 
 ValueBox<bool> j ( true);
 cout << j.str() << endl;
 
 ValueBox<int> i(100);
 cout << i.str() << endl;
 
 ValueBox<float> f ( 10.6f);
 cout << f.str() << endl;
 
 ValueBox<Box> b1 (Box());
 cout << b1.str() << endl;
 
 ValueBox<Bin> b2 (Bin());
 cout << b2.str() << endl;
 
 return 1;
 }

报错的原因是C++ 编译器把ValueBox<Box> b1 (Box());中的b1看成的函数定义，把Box()看成是返回值为Box的无参数函数，只不过此时的函数名称未定义罢了。这种错误有一个通用的名称，叫做：Most Vexing Parse

修改的方法有如下几种：

1、将Box单独放一行并初始化 
Box b;   // 不能使用Box b();，这样b又会被当成函数定义了 
ValueBox<Box> b1 (b);

2、将Box()用括号包起来，这样Box()就不会被看成是函数了 
ValueBox<Box> b1 ((Box()));

3、使用C++11中的统一初始化语法，即使用\{\} 
ValueBox<Box> b1 \{Box()\};

**参考文档**  
[C++语法分析中最让人头疼的歧义][C]  
[Item 6. Be alert for C++'s most vexing parse][Item 6. Be alert for C_s most vexing parse]  
[Effective STL 笔记: Item 6--Be alert for C++'s most vexing parse][Effective STL _ Item 6--Be alert for C_s most vexing parse]

[C]: http://tieba.baidu.com/p/2948104884
[Item 6. Be alert for C_s most vexing parse]: https://blog.csdn.net/dengzhilong_cpp/article/details/7098714
[Effective STL _ Item 6--Be alert for C_s most vexing parse]: https://www.cnblogs.com/yangyingchao/p/3394146.html