Why Does C Use the Scope Resolution Operator (::) While Java Doesn't?

Why C Introduced the Scope Resolution Operator

In comparison to Java, C provides a distinct scope resolution operator (::) instead of relying solely on the dot (.) operator. Despite Java's success without a separate operator, C necessitated this addition due to a unique language feature: the ability to use the same identifier for both a member variable and a derived class type.

This distinction becomes apparent when considering code like the example below:

struct foo { int blah; };
struct thingy { int data; };
struct bar : public foo { thingy foo; };

In this scenario, both the member variable foo within the bar struct and the derived class type foo share the same name. To resolve this ambiguity, the scope resolution operator :: is employed.

When the compiler encounters a ., it assumes the left-hand operand is an object. In contrast, :: signifies a typename, namespace, or the global namespace. This differentiation enables the compiler to interpret code like the following:

test.foo.data = 5;
test.foo::blah = 10;

In the first statement, test.foo.data is recognized as accessing the data member of the object test.foo. In the second statement, test.foo::blah denotes accessing the blah member of the derived class foo.

Thus, the scope resolution operator in C serves as a precision tool for resolving ambiguities created by the allowance of identical identifiers for member variables and derived class types.

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