为什么将NSString对象传递给XCTAssertTrue的“format”参数会导致构建错误？

Question

In attempting to use XCTest to test my application, I get a build error when doing the following:

#import <XCTest/XCTest.h>

@interface MyTests : XCTestCase

@end

@implementation MyTests

- (void)testExample
{
    NSString *str = @"foo";
    XCTAssertTrue(YES, str); // Parse issue: Expected ')'
}

@end

but I do not get a build error if I do this:

#import <XCTest/XCTest.h>

@interface MyTests : XCTestCase

@end

@implementation MyTests

- (void)testExample
{
    XCTAssertTrue(YES, @"foo"); // this is just fine...
}

@end

The build error that I get is:

Parse issue: Expected ')' 

and it puts an arrow under the "s" in "str".

I discovered that I can remedy this by changing

XCTAssertTrue(YES, str)

to

XCTAssertTrue(YES, @"%@", str)

but I just cannot figure out why that makes a difference. Can somebody please explain why this is so?

Answer 1

The XCT... macros are written to accept format strings — the strings themselves are optional (so that writing XCTAssertTrue(YES) is valid), but they must be constant strings. You cannot pass objects into the macro without having a format string, which is why XCTAssertTrue(YES, @"%@", str) works, but, say, XCTAssertTrue(YES, str) or XCTAssertTrue(NO, nil) wouldn't.

Answer 2

Deep inside the implementation, the code does this:

    @"" format

If format is a constant string literal, the compiler concatenates the strings. If format is anything else, you get a compiler error.

Answer 3

Passing predefined text into the assertion is sometimes desirable so:

XCTAssertTrue(YES, @"foo"); // this is just fine...

As is this

#define FOO @"foo"
XCTAssertTrue(YES, FOO); // this is just fine too...

So I do stuff like:

#define DBUEqualityTestFailed @"Equality test failed"

// test
DBNumber *n1 = [@((int)1) dbNumberFromIntValue];

XCTAssertTrue(*(int *)[n1 valuePointer] == 1, DBUEqualityTestFailed);
XCTAssertTrue([n1 valuePointer] == [n1 valuePointer], DBUEqualityTestFailed);
XCTAssertTrue(*(int *)[n1 valuePointer] == *(int *)[n1 valuePointer], DBUEqualityTestFailed);