编译器错误消息与cout

Question

我以前输错了错误信息.现在修好了.

我目前收到以下编译器错误消息

error: no match for 'operator<<' in 'std::cout << Collection::operator[](int)(j)'

编译器抱怨的代码是

cout << testingSet[j];

哪个testingSet类型的对象Collection已operator[]重载以返回类型的对象Example. Example有一个operator<<为ostream和Example 重载的友元函数.

注意:这实际上在Visual Studio中编译得很好; 但是不使用g ++编译.

这是执行operator<<:

ostream& operator<<(ostream &strm, Example &ex)
{
     strm << endl << endl;
     strm << "{ ";
     map<string, string>::iterator attrib;
     for(attrib = ex.attributes.begin(); attrib != ex.attributes.end(); ++attrib)
     {
          strm << "(" << attrib->first << " = " << attrib->second << "), ";
     }
     return strm << "} classification = " << (ex.classification ? "true" : "false") << endl;
}

而且 operator[]

Example Collection::operator[](int i)
{
      return examples[i];
}

Answer 1

可能您的运营商应声明为:

ostream& operator<<(ostream &strm, const Example &ex)

注意const-reference to Example.Visual Studio有一个扩展,允许绑定对非const r值的引用.我的猜测是你operator[]返回一个r值.

无论如何,operator<<应该是const因为不希望修改书面对象.