Cedar Expectations

November 2, 2011 Adam Milligan

As I wrote here we’ve used OCHamcrest matchers for some time for writing expectations in Cedar, but have found them unsatisfying. We wanted convenient matchers, like the ones Jasmine provides for JavaScript, but for Objective C. To that end, we added Cedar-specific expectation functions and matchers that specifically solve the problems we had with OCHamcrest.

To use Cedar’s expectations you need to make a couple small changes to your spec files:

1) Cedar matchers use C++ templates. Tell the compiler to expect some C++ code by changing the file extension on your spec files from .m to .mm.

2) Cedar matchers live in a C++ namespace. At the top of your spec file, after the includes, add this line:

using namespace Cedar::Matchers;


Type Deduction

Cedar matchers determine the types of the values in the expectation, so you don’t have to. Rather than this:

assertThat(someObject, equalTo(anotherObject));
assertThatUnsignedInt(someUInt, equalToUnsignedInt(anotherUInt));

you write this:


We can also mix and match object and non-object types, in cases where such comparisons make sense. For instance, rather than this:

NSNumber *aNumber = [NSNumber numberWithFloat:1.7];
assertThat(aNumber, equalTo([NSNumber numberWithFloat:1.8));
assertThatFloat([aNumber floatValue], equalToFloat(1.8));  // equivalent

you write this:

NSNumber *aNumber = [NSNumber numberWithFloat:1.7];


Cedar’s expectations use C++ templates not just for argument types, but for matcher objects as well. This means what goes inside the to() construct (e.g. expect(foo).to(equal(bar));) can be any object that responds to the appropriate methods (more on this later). If you want to write a custom matcher for your system (e.g. expect(user).to(be_logged_in());), no need to subclass anything or link against the Cedar library.

We also separated comparators from matchers for common expectations, such as equality. This means that adding equality comparison for a custom type involves only adding a comparator function for your type, with no need to modify the equality matcher itself. For example, here is the method for comparing two NSNumber objects:

bool compare_equal(NSNumber * const actualValue, NSNumber * const expectedValue) {
    return [actualValue isEqualToNumber:expectedValue];


Several versions of LLVM have had trouble with properly compiling Objective C++ code that contains blocks, and GCC has never done so correctly. Versions of LLVM before 2.0 (Xcode 3.x) will fail to compile Cedar specs with the Internal Compiler Error” message. Version 2.0 of LLVM (Xcode 4.0.x) will successfully compile Cedar specs, but incorrectly handle reference to temporaries, leading to difficult to understand error messages. Versions 2.1 (Xcode 4.1) and 3.0 (Xcode 4.2) fix this problem, and will work properly. However, most version of Xcode still default to using the GCC/LLVM hybrid, so you may need to change this setting.

Also, C++ templates deduce types at compile time, which means they use the matcher function for the declared type of parameters. For instance, if you have an equality comparator specialized for MyType, which is a subclass of NSObject, the following code will invoke the equality comparator for NSObject, not MyType:

NSObject *foo = [[MyType alloc] init]; // refers to a MyType, but is declared as NSObject *

In practice this seems to cause problems less frequently than I had expected. However, keep in mind that the declared type of nearly all Objective C initializers is id (NSNumber seems to be the exception to this rule, for some reason). So, if you pass the temporary result of an initializer to a matcher, it will most likely use the specialization of that matcher for id. For instance:

expect([NSString string]).to(equal(@""));  // Compares id to NSString * (works fine)
expect([[MyType alloc] init]).to(equal(@"")); // Compares id to NSString * (might work?)

Finally, because of the way C++ template work, the order of declaration of functions does matter. For instance, the equality matcher delegates comparison to the appropriate overload of the compare_equal function; this allows easy addition of custom comparators, as mentioned above. However, at the point where the compiler finds the equality matcher, it will only match the parameter types to comparator function overloads it has already seen. This mean you need to include your custom comparators before the declaration of the equality matcher. Since Cedar helpfully includes all matchers and comparators, you can do this in one of a few ways:

  1. Import your custom comparators before Cedar’s SpecHelper.h at the top of your file. This should work fine, since your custom matchers and comparators need not included dependencies from Cedar.

  2. Cedar checks a preprocessor value named CEDAR_CUSTOM_COMPARATORS. If you set this value Cedar will treat the value as a file path and #import it before the matcher library. Cedar also have CEDAR_CUSTOM_MATCHERS and CEDAR_CUSTOM_STRINGIFIERS (for providing custom output formats for your types).

  3. For new matchers and comparators for Cocoa types (CGRect, UIView, NSIndexPath, AVPlayer, etc.), feel free to contribute back to Cedar so they get included by default.

Please send any comments, questions, suggestions, feature request, pull requests, etc. to the Cedar discussion mailing list. Unfortunately, messages to the Pivotal account on GitHub get lost in the jillions of messages and notifications we receive.

About the Author


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