How to use the #error directive - C++

Question

I'm creating a self initializing arrays class in C++ and I'm wondering how I'd throw an error not an exception if a user were to try and allocate more than 0x7fffffff bytes.

Similar to <array> where:

error C2148: total size of array must not exceed 0x7fffffff bytes

This is my code for one of my constructor where I'm testing this:

 template<typename T> Array<T>::Array(const size_t _SIZE) : _SIZE(_SIZE), _content(nullptr){ #define __SIZE__ _SIZE #if (__SIZE__ > 0x7fffffff) #error Total size of Array must not exceed 0x7fffffff bytes. #endif _content = new T[_SIZE]; memset(_content, 0, (sizeof(_content) * _SIZE)); }

The way that I'm creating the array is below:

Array<int> foo(-1) //-1 of size_t = ((2^31)*2)-1 error should be shown since ((2^31)*2)-1 > ((2^31)*2)-1

size_t's max size is ((2^31)*2)-1 and 0x7fffffff is (2³¹)-1 now the issue is that the error isn't executing I've never used the #if macro before and I need to get this to work...

Any help would be appreciated.

I believe the memset line should look like this: memset(_content, 0, (sizeof(T) * _SIZE)); — user1764961
– user1764961, Commented May 15, 2013 at 6:46
Don't use identifiers that have double underscores or start with underscore followed by a capital. Those are reserved for the implementation (besides from looking ugly anyways), even if used before compilation. — Arne Mertz
– Arne Mertz, Commented May 15, 2013 at 6:56
@user1764961 Or sizeof(*_content). However, it T is not a primitive type, then memset should not be used at all, since it can to bad things to objects data, including the virtual table if the class have virtual function. — Some programmer dude
– Some programmer dude, Commented May 15, 2013 at 7:03
By the way, how is you class different from e.g. std::vector? You can set both the size and the initial data when constructing a std::vector. — Some programmer dude
– Some programmer dude, Commented May 15, 2013 at 7:05

Some programmer dude · Accepted Answer · 2013-05-15 07:13:01Z

You can't use the preprocessor for variables. The preprocessor is a separate step that is run before the compilation, and it has no idea about the variables used in the source and especially their run-time values.

For this you might want to use assert:

assert(_SIZE <= 0x7fffffff);

If you pass a negative value to a function expecting unsigned values then you should get a compiler warning, and if not then you should enable more warnings.

Oswald · Accepted Answer · 2013-05-15 06:46:47Z

#error is a preprocessor directive. As such, it is executed by the preprocessor during compilation. It cannot check what value is passed as _SIZE parameter because that value is only know when your programm is being executed.

Use some other means to signal errors during run time, e.g. exceptions.

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