编译器错误:“初始化元素不是编译时常量" [英] Compiler error: "initializer element is not a compile-time constant"
问题描述
在编译这段代码时,我收到错误初始化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中.这意味着您只能使用常量值.由于您在编译时并不了解有关运行时环境的所有信息(哪些类可用,它们的结构是什么等),因此在运行时之前您无法创建目标 c 对象,但常量字符串除外,它们被赋予特定的结构并保证保持这种状态.您应该做的是将变量初始化为 nil 并使用 +initialize
来创建您的图像.initialize
是一个类方法,它将在您的类上调用任何其他方法之前调用.
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.
示例:
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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