Cocoa Objective-c Property C 结构赋值失败 [英] Cocoa Objective-c Property C structure assign fails

问题描述

我想更改作为另一个类结构成员的变量值.但价值并没有改变.

I want to change the variable value which is a member of a structure of another class. But the value is not getting changed.

这是代码.

//Structure..

typedef struct {
    int a;
    double b;
} SomeType;


//Class which has the structure as member..
@interface Test2 : NSObject {
    // Define some class which uses SomeType
    SomeType member;

}

@property SomeType member;

@end


@implementation Test2

@synthesize member;

@end


//Tester file, here value is changed..
@implementation TesstAppDelegate

@synthesize window;

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
    // Insert code here to initialize your application 

    Test2 *t = [[Test2 alloc]init];

    t.member.a = 10;
//After this the value still shows 0

}

@end

我尝试使用以下链接.

Objective C 中类成员的结构

问候，达纳.

推荐答案

要更改成员"实例变量，您需要对其进行整体设置.您应该执行以下操作:

To make a change to your 'member' instance variable, you need to set it in its entirety. You should do something like:

SomeType mem = t.member;
mem.a = 10;
t.member = mem;

问题在于 t.member 被用作getter"(因为它后面没有紧跟一个 '=')，所以 t.member.a = 10; 等同于 [t member].a = 10;

The problem is that t.member is being used as a "getter" (since it's not immediately followed by an '='), so t.member.a = 10; is the same as [t member].a = 10;

那不会完成任何事情，因为 [t member] 返回一个结构体，它是一个r-value"，即.一个仅适用于赋值右侧的值.它有一个价值，但试图改变那个价值是没有意义的.

That won't accomplish anything, because [t member] returns a struct, which is an "r-value", ie. a value that's only valid for use on the right-hand side of an assignment. It has a value, but it's meaningless to try to change that value.

基本上，t.member 正在返回您的成员"结构的副本.然后您立即修改该副本，并在该方法结束时丢弃该副本.

Basically, t.member is returning a copy of your 'member' struct. You're then immediately modifying that copy, and at the end of the method that copy is discarded.

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