PricingLoginStart free
C++ Academy · Lesson

Assertions and Matchers

Write expressive checks.

Lesson 2 of 413 steps

Assertions and Matchers is a free C++ Academy lesson on CoddyKit. This is lesson 2 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the C++ Academy learning path, and your progress syncs across the web and the CoddyKit app. The C++ Academy course includes 4 lessons in total.

Assertions Express Expectations

GTest assertions check conditions inside a test. They come in two severities.

  • EXPECT_*: failure is recorded, test continues
  • ASSERT_*: failure stops the current test immediately

EXPECT vs ASSERT

Use ASSERT_* when continuing would crash (e.g. after a null check), and EXPECT_* otherwise so you see all failures at once.

#include <gtest/gtest.h>

TEST(Demo, MixedChecks) {
    int* p = new int(5);
    ASSERT_NE(p, nullptr);
    EXPECT_EQ(*p, 5);
    delete p;
}

Comparison Assertions

The core comparison macros:

  • EXPECT_EQ, EXPECT_NE
  • EXPECT_LT, EXPECT_LE
  • EXPECT_GT, EXPECT_GE
#include <gtest/gtest.h>

TEST(Compare, Numbers) {
    EXPECT_LT(2, 3);
    EXPECT_GE(5, 5);
}

Boolean Assertions

EXPECT_TRUE and EXPECT_FALSE check boolean conditions directly.

#include <gtest/gtest.h>

bool is_even(int n) { return n % 2 == 0; }

TEST(Bool, Even) {
    EXPECT_TRUE(is_even(4));
    EXPECT_FALSE(is_even(3));
}

Floating-Point Comparisons

Never use EXPECT_EQ on floats. Use EXPECT_FLOAT_EQ, EXPECT_DOUBLE_EQ, or EXPECT_NEAR with a tolerance.

#include <gtest/gtest.h>

TEST(Floats, Approx) {
    EXPECT_DOUBLE_EQ(0.1 + 0.2, 0.3);
    EXPECT_NEAR(3.14159, 3.14, 0.01);
}

String Assertions

For C strings use EXPECT_STREQ / EXPECT_STRNE. For std::string, EXPECT_EQ works since it compares by value.

#include <gtest/gtest.h>
#include <string>

TEST(Strings, Compare) {
    EXPECT_STREQ("hi", "hi");
    EXPECT_EQ(std::string("ok"), "ok");
}

Exception Assertions

Verify that code throws the expected exception.

#include <gtest/gtest.h>
#include <stdexcept>

void check(int n) {
    if (n < 0) throw std::runtime_error("neg");
}

TEST(Throws, Negative) {
    EXPECT_THROW(check(-1), std::runtime_error);
    EXPECT_NO_THROW(check(1));
}

Custom Failure Messages

Stream extra context into an assertion with << to make failures easier to diagnose.

#include <gtest/gtest.h>

TEST(Message, WithContext) {
    int x = 7;
    EXPECT_EQ(x, 7) << "x had unexpected value " << x;
}

GMock Matchers with EXPECT_THAT

EXPECT_THAT combines a value with a matcher for expressive checks, e.g. EXPECT_THAT(v, ElementsAre(1, 2, 3)).

#include <gtest/gtest.h>
#include <gmock/gmock.h>
#include <vector>
using ::testing::ElementsAre;

TEST(Matchers, Vector) {
    std::vector<int> v{1, 2, 3};
    EXPECT_THAT(v, ElementsAre(1, 2, 3));
}

Useful Matchers

Common matchers include:

  • Eq, Ne, Gt, Lt
  • Contains, ElementsAre, UnorderedElementsAre
  • HasSubstr, StartsWith, EndsWith

Choosing the Right Assertion

Prefer the most specific assertion: it produces clearer failure messages than a generic EXPECT_TRUE. EXPECT_EQ(a, b) prints both values; EXPECT_TRUE(a == b) does not.

Quick Check

Recall the EXPECT vs ASSERT difference.

Answer it in the interactive lesson

Recap

You learned GTest assertions.

  • EXPECT_* continues; ASSERT_* aborts the test
  • Comparison, boolean, float, string, exception macros
  • Stream extra context with <<
  • EXPECT_THAT + matchers for expressive checks
Free to start

Learn C++ with an AI tutor — free

Write and run real code in your browser, get instant help from a 24/7 AI tutor, and pick up where you left off on the web or in the app.

Courses
51
Lessons
203
Start learning freeView the full course

Frequently Asked Questions

Is the “Assertions and Matchers” lesson free?

Yes — any 3 lessons of the C++ Academy learning path, including “Assertions and Matchers”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The C++ Academy course includes 4 lessons in total.

What will I learn in “Assertions and Matchers”?

Write expressive checks. You practise C++ Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C++ Academy?

No prior experience is required. C++ Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 2 of 4.

How long does the “Assertions and Matchers” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this C++ Academy lesson?

Yes. Every C++ Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Setting Up GTest
  2. Assertions and Matchers
  3. Fixtures and Setup
  4. Mocking with GMock
← Back to C++ Academy