What does static_assert do, and what would you use it for?

by

Static assert is used to make assertions at compile time. When the static assertion fails, the program simply doesn’t compile. This is useful in different situations, like, for example, if you implement some functionality by code that critically depends on unsigned int object having exactly 32 bits. You can put a static assert like this

static_assert(sizeof(unsigned int) * CHAR_BIT == 32);

in your code. On another platform, with differently sized unsigned int type the compilation will fail, thus drawing attention of the developer to the problematic portion of the code and advising them to re-implement or re-inspect it.

For another example, you might want to pass some integral value as a void * pointer to a function (a hack, but useful at times) and you want to make sure that the integral value will fit into the pointer

int i;

static_assert(sizeof(void *) >= sizeof i);
foo((void *) i);

You might want to asset that char type is signed

static_assert(CHAR_MIN < 0);

or that integral division with negative values rounds towards zero

static_assert(-5 / 2 == -2);

And so on.

Run-time assertions in many cases can be used instead of static assertions, but run-time assertions only work at run-time and only when control passes over the assertion. For this reason a failing run-time assertion may lay dormant, undetected for extended periods of time.

Of course, the expression in static assertion has to be a compile-time constant. It can’t be a run-time value. For run-time values you have no other choice but use the ordinary assert.

More Related Contents:

  1. static_assert fails compilation even though template function is called nowhere
  2. C++11 – static_assert within constexpr function?
  3. “unpacking” a tuple to call a matching function pointer
  4. Why does reading a record struct fields from std::istream fail, and how can I fix it?
  5. Does C++11 allow vector?
  6. shared_ptr to an array : should it be used?
  7. Compiling C++11 with g++
  8. How much is too much with C++11 auto keyword?
  9. Why do I get the same sequence for every run with std::random_device with mingw gcc4.8.1?
  10. How do I pass a unique_ptr argument to a constructor or a function?
  11. What does std::match_results::size return?
  12. Where is the lock for a std::atomic?
  13. What are the rules for the “…” token in the context of variadic templates?
  14. Difference between C++03 throw() specifier C++11 noexcept
  15. Why do some people use swap for move assignments?
  16. Default, value and zero initialization mess
  17. How can I output the value of an enum class in C++11
  18. What exactly is a ‘side-effect’ in C++?
  19. How Does std::enable_if work?
  20. Vector of std::function with different signatures
  21. decltype as a return type in class member function
  22. How to enforce copy elision, why it won’t work with deleted copy constructor?
  23. Why are std::atomic objects not copyable?
  24. Is it smart to replace boost::thread and boost::mutex with c++11 equivalents?
  25. The implementation of std::forward
  26. Move or Named Return Value Optimization (NRVO)?
  27. Default Move Constructor in Visual Studio 2013 (Update 3)
  28. What is the purpose of std::function and how to use it?
  29. How does std::forward receive the correct argument?
  30. Overload resolution with ref-qualifiers

Leave a Comment