Game Engine Architecture

114 3. Fundamentals of Software Engineering for Games

// This ‘gInternalVariable’ is distinct from the one

// defined in foo.cpp – no error. We could just as

// well have named it gInternalVariableForBarCpp – the

// net effect is the same.

static U32 gInternalVariable;

// This function is distinct from foo.cpp’s

// version – no error. It acts as if we had named it

// internalFunctionForBarCpp().

static void internalFunction()

{

// ...

}

// ERROR – multiply defined symbol!

void externalFunction()

{

// ...

}

Technically speaking, declarations don’t have a linkage property at all, be-

cause they do not allocate any storage in the executable image; therefore, there

is no question as to whether or not the linker should be permitt ed to cross-

reference that storage. A declaration is merely a reference to an entity defi ned

elsewhere. However, it is sometimes convenient to speak about declarations

as having internal linkage, because a declaration only applies to the transla-

tion unit in which it appears. If we allow ourselves to loosen our terminology

in this manner, then declarations always have internal linkage—there is no

way to cross-reference a single declaration in multiple .cpp fi les. (If we put a

declaration in a header fi le, then multiple .cpp fi les can “see” that declaration,

but they are in eff ect each gett ing a distinct copy of the declaration, and each

copy has internal linkage within that translation unit.)

This leads us to the real reason why inline function defi nitions are permit-

ted in header fi les: It is because inline functions have internal linkage by de-

fault, just as if they had been declared static. If multiple .cpp fi les #include

a header containing an inline function defi nition, each translation unit gets a

private copy of that function’s body, and no “multiply defi ned symbol” errors

are generated. The linker sees each copy as a distinct entity.

3.2.3. C/C++ Memory Layout

A program writt en in C or C++ stores its data in a number of diff erent places in

memory. In order to understand how storage is allocated and how the various