Teague Teague - 1 month ago 9
C Question

Does "size_t" make code portable, or just 'more' portable?

I'm struggling to understand the usefulness of the C++ std::size_t data type. I realize that this data type is platform dependent, and is supposedly meant to make code more portable. However, it seems like it doesn't solve all the problems.

Say for example I'm working on a machine that has 32 bit int. Let's say I decide to write a c style function of this machine that just copies the bytes from one object to another. Inside this function, the memcpy function is used to write the data from object2 to object1. I've chosen an arbitrarily large number.

void writeBytes(obj *pobj1, obj *pobj2)
{
memcpy(pobj1, pobj2, 1048575);
}


This code should (hopefully) compile just fine. Because memcpy uses size_t in its declaration, and because size_t on this platform should be 32 bits, the number of 1048575 should work just fine.

But now let's say I decide to port this function over to a machine that 16 bit ints. Now the memcpy function interprets size_t as being of size 16. In this case, 1048575 is exceeds the allowed values for what memcpy was declared for. The code then fails to compile.

So my question: how exactly was size_t useful in this case? How did it make our code more portable?

Answer

size_t is able to hold the size of the largest object you can create. It is not required to be your platform's largest native integer type.

Given your example, your code would work regardless of the native integer being 16-bit or 32-bit if your platform allows 1048575 byte objects. Or the inverse -- if 1048575 doesn't fit in a size_t, you never could have created an object that large to memcpy.

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