What happens if a C++ assertion fails?
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When writing C++ code, we often encounter situations where we need to verify certain conditions. Assertions are a powerful tool for this purpose. Let’s find out what assertions are, how they work, and what happens if a C++ assertion fails?
What Are Assertions?
An assertion is like a safety net for your code. It allows you to express assumptions about your program and catch unexpected issues during development. When an assertion fails, it indicates that something isn’t as expected, helping you identify and fix problems early.
How Do Assertions Work?
In C++, if an assertion fails, it typically indicates that the program has encountered an unexpected condition. Here’s what happens in detail:
- Assertion Macro:
The assert macro, defined in the <cassert> header, is commonly used for assertions. The macro takes an expression as an argument.
- Condition Check:
When the program reaches an assert statement, the expression provided to assert is evaluated.
- Failure:
If the expression evaluates to false (i.e., the condition fails), the assert macro triggers an assertion failure.
- Error Message and Abort:
An error message is printed to the standard error stream (stderr). This message typically includes the file name, line number, and the condition that failed.
The program is then terminated by calling the abort function. This results in the termination of the program, often producing a core dump (on systems that support core dumps).
Here is an example of how assert is used:
#include <cassert>int main() {
int x = 5;
assert(x == 5); // This assertion will pass
assert(x == 0); // This assertion will fail
return 0;
}
When the second assertion fails, the output will look something like this:
a.out: main.cpp:5: int main(): Assertion `x == 0' failed.
Aborted (core dumped)
Important Considerations:
- Debugging Tool:
Assertions are primarily used as a debugging tool. They help catch logic errors and incorrect assumptions during development.
- Disabled in Release Builds
Assertions can be disabled in release builds by defining the NDEBUG macro before including the <cassert> header. This prevents assertions from being evaluated, allowing for potentially faster execution in production environments.
For example:
#define NDEBUG
#include <cassert>
In conclusion, an assertion failure in C++ causes the program to print an error message and abort execution, aiding developers in identifying and correcting issues during the development phase.
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