摆脱警告“表达结果未使用”

Question

I have the following few lines of code:

NSURL *url = [NSURL URLWithString:URL];
NSURLRequest* request = [NSURLRequest requestWithURL:url .... ];
NSURLConnection* connection = [NSURLConnection alloc];
[connection initWithRequest:request delegate:self];

In the last line I get "Expression Result Unused" warning. Now, according to all the articles online I have read, this is the correct way to call a method, and the syntax is as advised to download a URL async. How to rewrite this code to fix the warning?

Answer 1

The problem comes from the fact that method NSURLRequest initWithRequest… return an object that you don't store.

If you don't need it, you should write:

(void)[connection initWithRequest:request delegate:self];

On Xcode, you can use qualifier __unused to discard warning too:

__unused [connection initWithRequest:request delegate:self];

to inform the compiler that you deliberately want to ignore the returned value.

Answer 2

You can use this line:

 [NSURLConnection connectionWithRequest:request delegate:self];

instead of:

NSURLConnection* connection = [NSURLConnection alloc];
[connection initWithRequest:request delegate:self];

Answer 3

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wunused-value"
NSURL *url = [NSURL URLWithString:URL];
NSURLRequest* request = [NSURLRequest requestWithURL:url .... ];
NSURLConnection* connection = [NSURLConnection alloc];
[connection initWithRequest:request delegate:self];
#pragma clang diagnostic pop

For a list of all the Clang Warnings you can suppress take a look here

Answer 4

Replace the last 2 lines with:

connection = [[NSURLConnection alloc] initWithRequest:request delegate:self];

The warning is useful because alloc can return a different object than init (for example, when you use NSArray , which employs class cluster pattern ).

In that case connection would be a reference to this "intermediate" object returned by alloc instead of a fully initialized instance returned by init .

Answer 5

只需将最后一行更改为：

connection = [connection initWithRequest:request delegate:self];