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

I use it to ensure my assumptions about compiler behaviour, headers, libs and even my own code are correct. For example here I verify that the struct has been correctly packed to the expected size.

struct LogicalBlockAddress
{
#pragma pack(push, 1)
    Uint32 logicalBlockNumber;
    Uint16 partitionReferenceNumber;
#pragma pack(pop)
};
BOOST_STATIC_ASSERT(sizeof(LogicalBlockAddress) == 6);

In a class wrapping stdio.h's fseek(), I have taken some shortcuts with enum Origin and check that those shortcuts align with the constants defined by stdio.h

uint64_t BasicFile::seek(int64_t offset, enum Origin origin)
{
    BOOST_STATIC_ASSERT(SEEK_SET == Origin::SET);

You should prefer static_assert over assert when the behaviour is defined at compile time, and not at runtime, such as the examples I've given above. An example where this is not the case would include parameter and return code checking.

BOOST_STATIC_ASSERT is a pre-C++0x macro that generates illegal code if the condition is not satisfied. The intentions are the same, albeit static_assert is standardised and may provide better compiler diagnostics.

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.

Off the top of my head...

#include "SomeLibrary.h"

static_assert(SomeLibrary::Version > 2, 
         "Old versions of SomeLibrary are missing the foo functionality.  Cannot proceed!");

class UsingSomeLibrary {
   // ...
};

Assuming that SomeLibrary::Version is declared as a static const, rather than being #defined (as one would expect in a C++ library).

Contrast with having to actually compile SomeLibrary and your code, link everything, and run the executable only then to find out that you spent 30 minutes compiling an incompatible version of SomeLibrary.

@Arak, in response to your comment: yes, you can have static_assert just sitting out wherever, from the look of it:

class Foo
{
    public: 
        static const int bar = 3;
};

static_assert(Foo::bar > 4, "Foo::bar is too small :(");

int main()
{ 
    return Foo::bar;
}

$ g++ --std=c++0x a.cpp
a.cpp:7: error: static assertion failed: "Foo::bar is too small :("