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CS253 Assertions

Overview

There are two sorts of assertions in C++:

• assert
• static_assert

They both come from <cassert>.

assert

assert is a preprocessor macro that is, essentially:

    void assert(bool condition) {
        if (!condition) {
            cerr << "assertion failed: name-of-condition\n"
            abort();
        }
    }

• It’s a runtime condition.
  • It has to be in executable code.
  • What would be a place that’s not in executable code?

assert example

#include <cassert>
#include <string>

void delete_file(const std::string &fname) {
    assert(!fname.empty());
    remove(fname.c_str());
}

int main() {
    delete_file("tempfile");
    delete_file("");
}

a.out: c.cc:5: void delete_file(const string&): Assertion `!fname.empty()' failed.
SIGABRT: Aborted

When to use assert

• Do not use assert for dealing with user errors.
• Only use it for “impossible” circumstances:
  • Make sure that a function argument, representing a size, is positive.
  • Make sure that a pointer argument isn’t null.
  • etc.

static_assert

static_assert is like assert, but:

• It’s a compile-time test. It “executes” at compile time.
  • It fails like a syntax error fails.
  • It doesn’t have to be in executable code.
    • might be in a class definition
    • might be outside of any function or class
• It takes two arguments:
  • a condition
  • an error message string (optional starting in C++17)

static_assert example

#include <iostream>
#include <cassert>

static_assert(-1 >> 1 == -1, "right shift must preserve sign");

int main() {
    std::cout << "Hello, world!\n";
    static_assert(sizeof(char)==1, "char must be 8 bits");
    return 0;
}

static_assert(sizeof(int)==3, "int must be 24 bits");

c.cc:12: error: static assertion failed: int must be 24 bits