Question

以下代码无法编译：

#include <iostream> 
#include <set>
#include <utility>
using namespace std; 

template <typename T1, typename T2>  
class meta {
   pair<T1,T2> xy_pair; 

   public: 
   /* meta(const T1 & t1, const T2 & t2) : 
    xy_pair.first(t1), 
    xy_pair.second(t2)
    {}*/
  meta() : 
    xy_pair.first(T1()), 
    xy_pair.second(T2())
    {}
  void print() const{
      cout << xy_pair.first << endl; 
      cout << xy_pair.second << endl; 
  }  

}; 

int main(){
 meta<int,int> met_xy; 

}

我收到以下编译错误：

[root@localhost STL]# g++ -std=c++0x sets.cpp -o sets
sets.cpp: In constructor ‘meta<T1, T2>::meta()’:
sets.cpp:16:16: error: expected ‘(’ before ‘.’ token
sets.cpp:16:16: error: expected ‘{’ before ‘.’ token
sets.cpp: In member function ‘void meta<T1, T2>::print() const’:
sets.cpp:20:29: error: expected primary-expression before ‘,’ token
sets.cpp:20:32: error: expected primary-expression before ‘>’ token
sets.cpp:20:33: error: expected primary-expression before ‘.’ token
sets.cpp:21:29: error: expected primary-expression before ‘,’ token
sets.cpp:21:32: error: expected primary-expression before ‘>’ token
sets.cpp:21:33: error: expected primary-expression before ‘.’ token

Answer 1

您不能像这样在初始化列表中初始化成员对象的各个子成员。做到这一点

meta() : xy_pair(T1(), T2()) {}

或只是

meta() {}

或者实际上完全省略了默认构造函数 - 编译器生成的构造函数会做同样的事情。

Answer 2

您的代码应该如下所示：

#include <utility>
#include <ostream>
#include <iostream>

template <typename T1, typename T2>  
class meta {
   std::pair<T1, T2> xy_pair;

public: 
    meta(const T1 & t1, const T2 & t2) : xy_pair(t1, t2) { }
    // note initialization of the whole member xy_pair, not its members
    meta() : xy_pair() { }

    void print() const {
       cout << xy_pair.first << endl; 
       cout << xy_pair.second << endl; 
    }
}; 

int main() {
   meta<int, int> meta_xy;
}

代码不使用std :: pair和constructors进行编译

2 个答案: