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

C开发者_Go百科ould you give an example where static_assert(...) ('C++11') would solve the problem in hand elegantly?

I am familiar with run-time assert(...). When should I prefer static_assert(...) over regular assert(...)?

Also, in boost there is something called BOOST_STATIC_ASSERT, is it the same as static_assert(...)?

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 :("

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.

BOOST_STATIC_ASSERT is a cross platform wrapper for static_assert functionality.

Currently I am using static_assert in order to enforce "Concepts" on a class.

example:

template <typename T, typename U>
struct Type
{
  BOOST_STATIC_ASSERT(boost::is_base_of<T, Interface>::value);
  BOOST_STATIC_ASSERT(std::numeric_limits<U>::is_integer);
  /* ... more code ... */
};

This will cause a compile time error if any of the above conditions are not met.

One use of static_assert might be to ensure that a structure (that is an interface with the outside world, such as a network or file) is exactly the size that you expect. This would catch cases where somebody adds or modifies a member from the structure without realising the consequences. The static_assert would pick it up and alert the user.

In absence of concepts one can use static_assert for simple and readable compile-time type checking, for example, in templates:

template <class T>
void MyFunc(T value)
{
static_assert(std::is_base_of<MyBase, T>::value, 
              "T must be derived from MyBase");

// ...
}

This doesn't directly answers the original question, but makes an interesting study into how to enforce these compile time checks prior to C++11.

Chapter 2 (Section 2.1) of Modern C++ Design by Andrei Alexanderscu implements this idea of Compile-time assertions like this

template<int> struct CompileTimeError;
template<> struct CompileTimeError<true> {};

#define STATIC_CHECK(expr, msg) \
{ CompileTimeError<((expr) != 0)> ERROR_##msg; (void)ERROR_##msg; } 

Compare the macro STATIC_CHECK() and static_assert()

STATIC_CHECK(0, COMPILATION_FAILED);
static_assert(0, "compilation failed");

The static_assert can be used to forbid the use of the delete keyword this way:

#define delete static_assert(0, "The keyword \"delete\" is forbidden.");

Every modern C++ developer may want to do that if he or she wants to use a conservative garbage collector by using only classes and structs that overload the operator new to invoke a function that allocates memory on the conservative heap of the conservative garbage collector that can be initialized and instantiated by invoking some function that does this in the beginning of the main function.

For example every modern C++ developer that wants to use the Boehm-Demers-Weiser conservative garbage collector will in the beginning of the main function write:

GC_init();

And in every class and struct overload the operator new this way:

void* operator new(size_t size)
{
     return GC_malloc(size);
}

And now that the operator delete is not needed anymore, because the Boehm-Demers-Weiser conservative garbage collector is responsible to both free and deallocate every block of memory when it is not needed anymore, the developer wants to forbid the delete keyword.

One way is overloading the delete operator this way:

void operator delete(void* ptr)
{
    assert(0);
}

But this is not recommended, because the modern C++ developer will know that he/she mistakenly invoked the delete operator on run time, but this is better to know this soon on compile time.

So the best solution to this scenario in my opinion is to use the static_assert as shown in the beginning of this answer.

Of course that this can also be done with BOOST_STATIC_ASSERT, but I think that static_assert is better and should be preferred more always.