由编译器优化的变量 [英] Variable optimized away by compiler

问题描述

我开始调试一些试图找出错误的代码。当我从调试器尝试 p tlEntries 时，我得到了

I started debugging some code attempting to find my mistake. When I attempt to p tlEntries from the debugger I get the

<变量优化远离编译器>

消息在 if 语句时停止。以下是我的代码：

message while stopped on the if statement. The following is my code:

NSArray *tlEntries = [[NSArray alloc] initWithArray:[self fetchJSONValueForURL:url]];
for (NSDictionary *info in tlEntries) 
{
    if ([info objectForKey:@"screen_name"] != nil)
         NSLog(@"Found %@ in the timeline", [info objectForKey:@"screen_name"]);
}

早期调试让我相信URL确实返回了有效的 NSArray ，但我不明白为什么 tlEntries 正在优化掉。

Earlier debugging gives me confidence the URL is indeed returning a valid NSArray, but I don't understand why tlEntries is being "optimized away".

推荐答案

编译器可能注意到您在开始时只使用了两次tlEntries，并且在循环中根本不使用它。如果我没记错的话，循环会创建一个枚举对象，而不是保持对容器对象的引用。所以tlEntries应该对第一行有效，但后来又用完了。

The compiler probably noticed that you only use tlEntries twice in the beginning, and don't use it at all in the loop. Loops create an enumeration object instead of keeping a reference to the container object, if I remember correctly. So tlEntries should be valid for the first line but then gone for the rest.

想法：你可以强制编译器通过在函数后面的某个地方使用tlEntries来保留它。类似

Idea: You can force the compiler to keep it by using tlEntries somewhere later in the function. Something like

NSPrint(@"IGNORE THIS LINE %p", tlEntries);

更好的想法：您可以将优化设置为-O0。强烈建议您调试代码。如果您使用Debug而不是Release版本，它应该自动设置为-O0，但您可以更改它。

Better idea: You can set the optimization to -O0. This is highly recommended for debugging code. It should be set to -O0 automatically if you use the "Debug" instead of "Release" builds, but you can change that.

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