Debugging with GDB: C Operators

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15.4.1.1 C and C++ Operators

Operators must be defined on values of specific types. For instance, + is defined on numbers, but not on structures. Operators are often defined on groups of types.

For the purposes of C and C++, the following definitions hold:

• Integral types include int with any of its storage-class specifiers; char; enum; and, for C++, bool.
• Floating-point types include float, double, and long double (if supported by the target platform).
• Pointer types include all types defined as (type *).
• Scalar types include all of the above.

The following operators are supported. They are listed here in order of increasing precedence:

,

The comma or sequencing operator. Expressions in a comma-separated list are evaluated from left to right, with the result of the entire expression being the last expression evaluated.

=

Assignment. The value of an assignment expression is the value assigned. Defined on scalar types.

op=

Used in an expression of the form a op= b , and translated to a = a op b . op= and = have the same precedence. The operator op is any one of the operators |, ^, &, <<, >>, +, -, *, /, %.

?:

The ternary operator. a ? b : c can be thought of as: if a then b else c. The argument a should be of an integral type.

||

Logical OR. Defined on integral types.

&&

Logical AND. Defined on integral types.

|

Bitwise OR. Defined on integral types.

^

Bitwise exclusive-OR. Defined on integral types.

&

Bitwise AND. Defined on integral types.

==, !=

Equality and inequality. Defined on scalar types. The value of these expressions is 0 for false and non-zero for true.

<, >, <=, >=

Less than, greater than, less than or equal, greater than or equal. Defined on scalar types. The value of these expressions is 0 for false and non-zero for true.

<<, >>

left shift, and right shift. Defined on integral types.

@

The GDB “artificial array” operator (see Expressions).

+, -

Addition and subtraction. Defined on integral types, floating-point types and pointer types.

*, /, %

Multiplication, division, and modulus. Multiplication and division are defined on integral and floating-point types. Modulus is defined on integral types.

++, --

Increment and decrement. When appearing before a variable, the operation is performed before the variable is used in an expression; when appearing after it, the variable’s value is used before the operation takes place.

*

Pointer dereferencing. Defined on pointer types. Same precedence as ++.

&

Address operator. Defined on variables. Same precedence as ++.

For debugging C++, GDB implements a use of ‘&’ beyond what is allowed in the C++ language itself: you can use ‘&(&ref)’ to examine the address where a C++ reference variable (declared with ‘&ref ’) is stored.

-

Negative. Defined on integral and floating-point types. Same precedence as ++.

!

Logical negation. Defined on integral types. Same precedence as ++.

~

Bitwise complement operator. Defined on integral types. Same precedence as ++.

., ->

Structure member, and pointer-to-structure member. For convenience, GDB regards the two as equivalent, choosing whether to dereference a pointer based on the stored type information. Defined on struct and union data.

.*, ->*

Dereferences of pointers to members.

[]

Array indexing. a[i] is defined as *(a+i). Same precedence as ->.

()

Function parameter list. Same precedence as ->.

::

C++ scope resolution operator. Defined on struct, union, and class types.

::

Doubled colons also represent the GDB scope operator (see Expressions). Same precedence as ::, above.

If an operator is redefined in the user code, GDB usually attempts to invoke the redefined version instead of using the operator’s predefined meaning.

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