C++ 错误 C2228：“.size”的左边必须有类/结构/联合
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error C2228: left of '.size' must have class/struct/union
提问by andandandand
I'm getting this compiler error when calling vector's size(). Why?
调用 vector 的size(). 为什么？
#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. \nEnter 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;
}
回答by In silico
Believe it or not, this line does notdeclare an instance of std::vectornamed results, calling the constructor taking a begin and end iterator:
信不信由你，这一行并没有声明一个std::vectornamed的实例，而是使用results一个开始和结束迭代器调用构造函数：
std::vector<float> results(std::istream_iterator<int>(actualLineStream),
std::istream_iterator<int>());
This actually declares a functioncalled resultsthat takes a parameter named actualLineStreamand another unnamed parameter, both of type std::istream_iterator<int>.
这实际上声明了一个被调用的函数results，它接受一个名为 的参数actualLineStream和另一个未命名的参数，两者都是类型std::istream_iterator<int>。
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.
通常在 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，以至于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。
It is related to the fact that these two lines are not equivalent in C++:
这与这两行在 C++ 中不等价的事实有关：
Foo bar; // Declares an instance of Foo named bar
Foo bar(); // Declares a function named bar that takes no parameters and returns a Foo
To fix it, you can add more parentheses around one of the arguments:
要修复它，您可以在其中一个参数周围添加更多括号：
// +--------- Note extra parentheses!! ---------+
// | |
// V V
std::vector<float> results((std::istream_iterator<int>(actualLineStream)),
std::istream_iterator<int>());
Or simply declare each iterator separately:
或者简单地分别声明每个迭代器：
std::istream_iterator<int> resultsBegin(actualLineStream);
std::istream_iterator<int> resultsEnd;
std::vector<float> results(resultsBegin, resultsEnd);
回答by mkb
I think you hit this bugaboo. You have managed to declare resultsas a new function that returns a std::vector<float>. If I change the line that declares resultsto:
我想你遇到了这个问题。您已成功声明results为一个返回std::vector<float>. 如果我将声明的行更改results为：
std::vector<float> results = std::vector<float>( std::istream_iterator<int>(actualLineStream), std::istream_iterator<int>() );
I can compile this (with GCC, though, YMMV)
我可以编译这个（不过，使用 GCC，YMMV）
