问题描述

编译此代码时，我收到错误初始化程序元素不是编译时常量".谁能解释一下为什么?

When compiling this code, I get the error "initializer element is not a compile-time constant". Can anyone explain why?

#import "PreferencesController.h"

@implementation PreferencesController

- (id)init
{
    self = [super init];
    if (self) {
        // Initialization code here.
    }

    return self;
}


NSImage* imageSegment = [[NSImage alloc] initWithContentsOfFile:@"/User/asd.jpg"];//error here

推荐答案

当您在函数范围之外定义变量时，该变量的值实际上会写入您的可执行文件.这意味着您只能使用常量值.由于您在编译时不了解运行时环境的所有信息(哪些类可用，它们的结构是什么等)，所以在运行时之前您无法创建目标 c 对象，但常量字符串除外，它被赋予特定的结构并保证保持这种状态.你应该做的是将变量初始化为 nil 并使用 +initialize 来创建你的图像.initialize 是一个类方法，它将在你的类上调用任何其他方法之前被调用.

When you define a variable outside the scope of a function, that variable's value is actually written into your executable file. This means you can only use a constant value. Since you don't know everything about the runtime environment at compile time (which classes are available, what is their structure, etc.), you cannot create objective c objects until runtime, with the exception of constant strings, which are given a specific structure and guaranteed to stay that way. What you should do is initialize the variable to nil and use +initialize to create your image. initialize is a class method which will be called before any other method is called on your class.

例子:

NSImage *imageSegment = nil;
+ (void)initialize {
    if(!imageSegment)
        imageSegment = [[NSImage alloc] initWithContentsOfFile:@"/User/asd.jpg"];
}
- (id)init {
  self = [super init];
  if (self) {
    // Initialization code here.
  }

  return self;
}

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