C++ 错误 - '.' 之前的预期主表达式 令牌|
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C++ Error - expected primary-expression before '.' token|
提问by Vaaljan
I just want to say that I am still learning C++ so I started with the module about Classes and Structures, and while I do not understand everything, I think I got it somewhat right. The error the compiler keeps giving me is:
我只想说我还在学习 C++，所以我从关于类和结构的模块开始，虽然我不是所有的东西，但我觉得我说得有点对。编译器不断给我的错误是：
error: expected primary-expression before '.' token
error: expected primary-expression before '.' token
Here is the Code:
这是代码：
#include <iostream>
using namespace std;
class Exam{
private:
string module,venue,date;
int numberStudent;
public:
//constructors:
Exam(){
numberStudent = 0;
module,venue,date = "";
}
//accessors:
int getnumberStudent(){ return numberStudent; }
string getmodule(){ return module; }
string getvenue(){ return venue; }
string getdate(){ return date; }
};
int main()
{
cout << "Module in which examination is written"<< Exam.module;
cout << "Venue of examination : " << Exam.venue;
cout << "Number of Students : " << Exam.numberStudent;
cout << "Date of examination : " << Exam.date
<< endl;
return 0;
}
The Question asked to use accessors and Mutators, But I don't know why I should use the Mutators.
问题要求使用访问器和变异器，但我不知道为什么我应该使用变异器。
Not 100% sure how they work anyways.
不是 100% 确定它们是如何工作的。
回答by LihO
In your class Exam: module, venueand dateare private members, which can be access only within the scope of this class. Even if you change the access modifier to public:
在你的class Exam：module，venue并且date是私有成员，这只能是这个类的范围内访问。即使您将访问修饰符更改为public：
class Exam {
public:
string module,venue,date;
}
those are still members that are associated with a concrete objects (instances of this class) rather than the class definition itself (like staticmembers would be). To use members of this kind, you need an object:
这些仍然是与具体对象（此类的实例）相关联的成员，而不是与类定义本身（就像static成员一样）。要使用此类成员，您需要一个对象：
Exam e;
e.date = "09/22/2013";
etc. Also note that module,venue,date = "";doesn't modify moduleand venuein any way, what you actually meant was:
此外，请注意module,venue,date = "";不修改module，并venue以任何方式，你实际的意思是：
module = venue = date = "";
although std::stringobjects are initialized to empty string automatically, thus this line is useless anyway.
尽管std::string对象会自动初始化为空字符串，因此无论如何这一行是无用的。
回答by Ranalee B.
You need the mutators function to accept input from the user to store in your variables module,venue and date
您需要 mutators 函数来接受来自用户的输入以存储在您的变量 module、venue 和 date 中
EXAMPLE:
例子：
void setdetails()
{
cin.ignore();
cout<<"Please Enter venue"<<endl;
getline(cin,venue);
cout<<"Please Enter module"<<endl;
getline(cin,module);
}//end of mutator function
