错误 C2228:“.size"的左边必须有类/结构/联合
我在调用 vector 的 size() 时遇到这个编译器错误.为什么?
I'm getting this compiler error when calling vector's size(). Why?
#include <vector>
#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <cassert>
using namespace std;
class Vertex {
float firstValue;
float secondValue;
float thirdValue;
Vertex (float first, float second, float third){
firstValue=first;
secondValue=second;
thirdValue=third;
}
};
int main()
{
cout<<"This program loads a 3D .off object.
Enter the name of the file that describes it "<<endl;
string inputFileName;
getline(cin, inputFileName);
ifstream inputFileStream;
inputFileStream.open(inputFileName.data());
assert (inputFileStream.is_open());
string actualLine;
for(;;){
inputFileStream>>actualLine;
istringstream actualLineStream(actualLine);
std::vector<float> results( std::istream_iterator<int>(actualLineStream)
, std::istream_iterator<int>() );
int resultsIndex=0;
int resultsSize=results.size(); //WHY??
while (resultsIndex<resultsSize){
cout<<results[resultsIndex]<<endl;
}
if (inputFileStream.eof()) break;
}
ofstream outputChannel;
while (true){} // to keep on console view
return 0;
}
推荐答案
信不信由你，这一行并没有声明一个名为std::vector的实例code>results，使用开始和结束迭代器调用构造函数:
Believe it or not, this line does not declare an instance of std::vector named results, calling the constructor taking a begin and end iterator:
std::vector<float> results(std::istream_iterator<int>(actualLineStream),
std::istream_iterator<int>());
这实际上声明了一个名为results的函数，它接受一个名为actualLineStream的参数和另一个未命名的参数，两者都是std类型::istream_iterator.
This actually declares a function called results that takes a parameter named actualLineStream and another unnamed parameter, both of type std::istream_iterator<int>.
一般在 C++ 中，如果某个东西看起来像一个函数，它就会像一个函数一样被解析；C++ 标准要求它.这实际上是为了与 C 向后兼容――但这太违反直觉了，它甚至有自己的名字:"最令人烦恼的解析".一些编译器甚至会在遇到最烦人的解析时发出警告.
Generally in C++, if something looks like a function, it will be parsed like one; the C++ standard requires it. This is really for backward compatibility with C - but this is so counterintuitive that it even has its own name: the "most vexing parse". Some compilers will even issue a warning if it encounters the most vexing parse.
这与这两行在 C++ 中不等价有关:
It is related to the fact that these two lines are not equivalent in C++:
Foo bar; // Declares an instance of Foo named bar
Foo bar(); // Declares a function named bar that takes no parameters and returns a Foo
要修复它，您可以在其中一个参数周围添加更多括号:
To fix it, you can add more parentheses around one of the arguments:
// +--------- Note extra parentheses!! ---------+
// | |
// V V
std::vector<float> results((std::istream_iterator<int>(actualLineStream)),
std::istream_iterator<int>());
或者简单地分别声明每个迭代器:
Or simply declare each iterator separately:
std::istream_iterator<int> resultsBegin(actualLineStream);
std::istream_iterator<int> resultsEnd;
std::vector<float> results(resultsBegin, resultsEnd);
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