7 Strict type checking

Both C99 and C++98 have about the same type checking strictness. Although the original K&R C has very loose type checking, the newer standards make C a strongly-typed language.

Generally speaking, C and C++ does not let you use one type when another type is expected. The exceptions to this rule can be summarized as follows:

• for scalars (like integers, real numbers and etc.):
  • It is okay to specify a scalar parameter with a smaller range when a scalar with a larger range is expected.
  • In a scalar assignment, the type of the left hand side can have a bigger range than the type of the right hand side.
  • The above exception does not work when passed by reference.
• for pointers:
  • (C++ only): It is okay to specify a pointer to a subclass when a pointer to a superclass is expected.
  • (C and C++): It is okay to specify a pointer to anything when a void pointer (void*) is expected.
• for all:
  • It is okay to specify a non-const object when a const object is expected.

Note that C and C++ both consider the following two named structures different:

struct X
{
};

struct Y
{
};

Despite both structures have no members (and hence structurally identical), X and Y are still considered different types. Consequently, the following code generates a warning:

struct X *pX;
struct Y *pY;

// ...
pX = pY;