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##### 15.4.1.3 C`++` Expressions

gdb expression handling can interpret most C`++` expressions.

> Warning: gdb can only debug C`++` code if you use the proper compiler and the proper debug format. Currently, gdb works best when debugging C`++` code that is compiled with gcc 2.95.3 or with gcc 3.1 or newer, using the options `-gdwarf-2` or `-gstabs+`. DWARF 2 is preferred over stabs+. Most configurations of gcc emit either DWARF 2 or stabs+ as their default debug format, so you usually don't need to specify a debug format explicitly. Other compilers and/or debug formats are likely to work badly or not at all when using gdb to debug C`++` code.

1. Member function calls are allowed; you can use expressions like

                 count = aml->GetOriginal(x, y)

2. While a member function is active (in the selected stack frame), your expressions have the same namespace available as the member function; that is, gdb allows implicit references to the class instance pointer `this` following the same rules as C`++`.

3. You can call overloaded functions; gdb resolves the function call to the right definition, with some restrictions. gdb does not perform overload resolution involving user-defined type conversions, calls to constructors, or instantiations of templates that do not exist in the program. It also cannot handle ellipsis argument lists or default arguments.

   It does perform integral conversions and promotions, floating-point promotions, arithmetic conversions, pointer conversions, conversions of class objects to base classes, and standard conversions such as those of functions or arrays to pointers; it requires an exact match on the number of function arguments.

   Overload resolution is always performed, unless you have specified `set overload-resolution off`. See gdb Features for C`++`.

   You must specify `set overload-resolution off` in order to use an explicit function signature to call an overloaded function, as in

                 p 'foo(char,int)'('x', 13)

   The gdb command-completion facility can simplify this; see Command Completion.

4. gdb understands variables declared as C`++` references; you can use them in expressions just as you do in C`++` source—they are automatically dereferenced.

   In the parameter list shown when gdb displays a frame, the values of reference variables are not displayed (unlike other variables); this avoids clutter, since references are often used for large structures. The address of a reference variable is always shown, unless you have specified ‘`set print address off`’.

5. gdb supports the C`++` name resolution operator `::`—your expressions can use it just as expressions in your program do. Since one scope may be defined in another, you can use `::` repeatedly if necessary, for example in an expression like ‘`scope1::scope2::name`’. gdb also allows resolving name scope by reference to source files, in both C and C`++` debugging (see Program Variables).

In addition, when used with HP's C`++` compiler, gdb supports calling virtual functions correctly, printing out virtual bases of objects, calling functions in a base subobject, casting objects, and invoking user-defined operators.