C++：从指针到类访问成员结构的语法
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C++: syntax for accessing member struct from pointer to class
提问by Peter ?lsted
I'm trying to access a member structs variables, but I can't seem to get the syntax right. The two compile errors pr. access are: error C2274: 'function-style cast' : illegal as right side of '.' operator error C2228: left of '.otherdata' must have class/struct/union I have tried various changes, but none successful.
我正在尝试访问成员结构变量，但我似乎无法获得正确的语法。两个编译错误pr。访问是：错误 C2274：“函数式强制转换”：“.”的右侧是非法的。操作员错误 C2228：'.otherdata' 的左边必须有类/结构/联合我尝试了各种更改，但都没有成功。
#include <iostream>
using std::cout;
class Foo{
public:
struct Bar{
int otherdata;
};
int somedata;
};
int main(){
Foo foo;
foo.Bar.otherdata = 5;
cout << foo.Bar.otherdata;
return 0;
}
回答by schnaader
You only define a struct there, not allocate one. Try this:
你只在那里定义一个结构，而不是分配一个。尝试这个：
class Foo{
public:
struct Bar{
int otherdata;
} mybar;
int somedata;
};
int main(){
Foo foo;
foo.mybar.otherdata = 5;
cout << foo.mybar.otherdata;
return 0;
}
If you want to reuse the struct in other classes, you can also define the struct outside:
如果要在其他类中重用该结构体，也可以在外部定义该结构体：
struct Bar {
int otherdata;
};
class Foo {
public:
Bar mybar;
int somedata;
}
回答by aJ.
Baris inner structure defined inside Foo. Creation of Fooobject does not implicitly create the Bar's members. You need to explicitly create the object of Bar using Foo::Barsyntax.
Bar是内部定义的内部结构Foo。创作Foo对象不隐式创建Bar的成员。您需要使用Foo::Bar语法显式创建 Bar 的对象。
Foo foo;
Foo::Bar fooBar;
fooBar.otherdata = 5;
cout << fooBar.otherdata;
Otherwise,
除此以外，
Create the Bar instance as member in Fooclass.
创建 Bar 实例作为Foo类中的成员。
class Foo{
public:
struct Bar{
int otherdata;
};
int somedata;
Bar myBar; //Now, Foo has Bar's instance as member
};
Foo foo;
foo.myBar.otherdata = 5;
回答by Peter ?lsted
You create a nested structure, but you never create any instances of it within the class. You need to say something like:
您创建了一个嵌套结构，但您从未在类中创建它的任何实例。你需要这样说：
class Foo{
public:
struct Bar{
int otherdata;
};
Bar bar;
int somedata;
};
You can then say:
然后你可以说：
foo.bar.otherdata = 5;
回答by stefanB
You are only declaring Foo::Bar but you don't instantiate it (not sure if that's the correct terminology)
你只是在声明 Foo::Bar 但你没有实例化它（不确定这是否是正确的术语）
See here for usage:
用法见这里：
#include <iostream>
using namespace std;
class Foo
{
public:
struct Bar
{
int otherdata;
};
Bar bar;
int somedata;
};
int main(){
Foo::Bar bar;
bar.otherdata = 6;
cout << bar.otherdata << endl;
Foo foo;
//foo.Bar.otherdata = 5;
foo.bar.otherdata = 5;
//cout << foo.Bar.otherdata;
cout << foo.bar.otherdata << endl;
return 0;
}
回答by Naveen
struct Bar{
int otherdata;
};
Here you have just defined a structure but not created any object of it. Hence when you say foo.Bar.otherdata = 5;it is compiler error. Create a object of struct Bar like Bar m_bar;and then use Foo.m_bar.otherdata = 5;
在这里，您刚刚定义了一个结构，但没有创建它的任何对象。因此，当您说foo.Bar.otherdata = 5;这是编译器错误时。创建一个 struct Bar 对象，Bar m_bar;然后使用Foo.m_bar.otherdata = 5;
