catch2/docs/assertions.md

Assertion Macros

Contents
Natural Expressions
Floating point comparisons
Exceptions
Matcher expressions
Thread Safety
Expressions with commas

Most test frameworks have a large collection of assertion macros to capture all possible conditional forms (_EQUALS, _NOTEQUALS, _GREATER_THAN etc).

Catch is different. Because it decomposes natural C-style conditional expressions most of these forms are reduced to one or two that you will use all the time. That said there is a rich set of auxiliary macros as well. We'll describe all of these here.

Most of these macros come in two forms:

Natural Expressions

The REQUIRE family of macros tests an expression and aborts the test case if it fails. The CHECK family are equivalent but execution continues in the same test case even if the assertion fails. This is useful if you have a series of essentially orthogonal assertions and it is useful to see all the results rather than stopping at the first failure.

• REQUIRE( expression ) and
• CHECK( expression )

Evaluates the expression and records the result. If an exception is thrown, it is caught, reported, and counted as a failure. These are the macros you will use most of the time.

Examples:

CHECK( str == "string value" );
CHECK( thisReturnsTrue() );
REQUIRE( i == 42 );

Expressions prefixed with ! cannot be decomposed. If you have a type that is convertible to bool and you want to assert that it evaluates to false, use the two forms below:

• REQUIRE_FALSE( expression ) and
• CHECK_FALSE( expression )

Note that there is no reason to use these forms for plain bool variables, because there is no added value in decomposing them.

Example:

Status ret = someFunction();
REQUIRE_FALSE(ret); // ret must evaluate to false, and Catch2 will print
                    // out the value of ret if possibly

Other limitations

Note that expressions containing either of the binary logical operators, && or ||, cannot be decomposed and will not compile. The reason behind this is that it is impossible to overload && and || in a way that keeps their short-circuiting semantics, and expression decomposition relies on overloaded operators to work.

Simple example of an issue with overloading binary logical operators is a common pointer idiom, p && p->foo == 2. Using the built-in && operator, p is only dereferenced if it is not null. With overloaded &&, p is always dereferenced, thus causing a segfault if p == nullptr.

If you want to test expression that contains && or ||, you have two options.

1. Enclose it in parentheses. Parentheses force evaluation of the expression before the expression decomposition can touch it, and thus it cannot be used.

2. Rewrite the expression. REQUIRE(a == 1 && b == 2) can always be split into REQUIRE(a == 1); REQUIRE(b == 2);. Alternatively, if this is a common pattern in your tests, think about using Matchers. instead. There is no simple rewrite rule for ||, but I generally believe that if you have || in your test expression, you should rethink your tests.

Floating point comparisons

Comparing floating point numbers is complex, and so it has its own documentation page.

Exceptions

• REQUIRE_NOTHROW( expression ) and
• CHECK_NOTHROW( expression )

Expects that no exception is thrown during evaluation of the expression.

• REQUIRE_THROWS( expression ) and
• CHECK_THROWS( expression )

Expects that an exception (of any type) is be thrown during evaluation of the expression.

• REQUIRE_THROWS_AS( expression, exception type ) and
• CHECK_THROWS_AS( expression, exception type )

Expects that an exception of the specified type is thrown during evaluation of the expression. Note that the exception type is extended with const& and you should not include it yourself.

• REQUIRE_THROWS_WITH( expression, string or string matcher ) and
• CHECK_THROWS_WITH( expression, string or string matcher )

Expects that an exception is thrown that, when converted to a string, matches the string or string matcher provided (see next section for Matchers).

e.g.

REQUIRE_THROWS_WITH( openThePodBayDoors(), Contains( "afraid" ) && Contains( "can't do that" ) );
REQUIRE_THROWS_WITH( dismantleHal(), "My mind is going" );

• REQUIRE_THROWS_MATCHES( expression, exception type, matcher for given exception type ) and
• CHECK_THROWS_MATCHES( expression, exception type, matcher for given exception type )

Expects that exception of exception type is thrown and it matches provided matcher (see the documentation for Matchers).

Please note that the THROW family of assertions expects to be passed a single expression, not a statement or series of statements. If you want to check a more complicated sequence of operations, you can use a C++11 lambda function.

REQUIRE_NOTHROW([&](){
    int i = 1;
    int j = 2;
    auto k = i + j;
    if (k == 3) {
        throw 1;
    }
}());

Matcher expressions

To support Matchers a slightly different form is used. Matchers have their own documentation.

• REQUIRE_THAT( lhs, matcher expression ) and
• CHECK_THAT( lhs, matcher expression )

Matchers can be composed using &&, || and ! operators.

Thread Safety

Currently assertions in Catch are not thread safe. For more details, along with workarounds, see the section on the limitations page.

Expressions with commas

Because the preprocessor parses code using different rules than the compiler, multiple-argument assertions (e.g. REQUIRE_THROWS_AS) have problems with commas inside the provided expressions. As an example REQUIRE_THROWS_AS(std::pair<int, int>(1, 2), std::invalid_argument); will fail to compile, because the preprocessor sees 3 arguments provided, but the macro accepts only 2. There are two possible workarounds.

1. Use typedef:

using int_pair = std::pair<int, int>;
REQUIRE_THROWS_AS(int_pair(1, 2), std::invalid_argument);

This solution is always applicable, but makes the meaning of the code less clear.

2. Parenthesize the expression:

TEST_CASE_METHOD((Fixture<int, int>), "foo", "[bar]") {
    SUCCEED();
}

This solution is not always applicable, because it might require extra changes on the Catch's side to work.

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