How to combine static_assert with sizeof and stringify?

user_163551 0
Viewed 13434

Memory usage is quite critical in my application. Therefore I have specific asserts that check for the memory size at compile time and give a static_assert if the size is different from what we considered correct before.

I have defined a macro like this:

#define CHECKMEM(mytype, size) static_assert((sizeof(objectType) == size)), "Size incorrect for " #mytype "!");

This macro makes it very easy to write this:

CHECKMEM(Book,144);
CHECKMEM(Library,80);

The problem is that when this static_assert goes off, it might be quite difficult to find out what the new size should be (e.g. by using the hidden compiler option "/d1 reportAllClassLayout"). It would be much handier if I could include the actual size, so instead of:

Size incorrect for Book!

It would show

Size incorrect for Book! (expected 144, size is 152)

I tried writing something like this:

#define CHECKMEM(mytype, size) static_assert((sizeof(objectType) == size)), "Size incorrect for " #mytype "! (expected" #size ", size is " #sizeof(mytype) ")");

But you can't use the stringize (#) operator on a function call.

I also tried adding the double-stringize trick, like this:

#define STR1(x) #x 
#define STR2(x) STR1(x) 
#define CHECKMEM(mytype, size) static_assert((sizeof(objectType) == size)), "Size incorrect for " #mytype "! (expected" #size ", size is " STR2(sizeof(mytype)) ")");

But instead of printing size is 152 it prints size is sizeof(Book).

Is there a way to stringify the result of sizeof in a static_assert?

stringify static-assert c++11 visual-studio-2010 c++
5 Answers

Here's an alternative header-only solution if you can modify the struct's definition a bit and don't mind some ugliness.

template <int RealSize = 0, int ExpectedSize = 0>
struct _MyStruct {
    static_assert(RealSize == ExpectedSize, "size is invalid");

    int x;
};

typedef _MyStruct<sizeof(_MyStruct<>), 4> MyStruct;

clang outputs:

prog.cpp:4:5: error: static_assert failed "size is invalid"
    static_assert(RealSize == ExpectedSize, "size is invalid");
    ^             ~~~~~~~~~~~~~~~~~~~~~~~~
prog.cpp:12:14: note: in instantiation of template class '_MyStruct<4, 8>' requested here
    MyStruct m;

One caveat here is that the check will only occur if you instantiate the type somewhere -- just using a pointer won't trigger the error, so definitely not a great fit for all situations!

user_54839040

A simple practical solution is to use 2 static_assert-s:

#define CHECKMEM(mytype, size) \
static_assert( sizeof(mytype) <= (size), "Size of " #mytype " is greater than " #size "!" ); \
static_assert( sizeof(mytype) >= (size), "Size of " #mytype " is less than "    #size "!" )
user_10959710
Related