In the past, the GNU C++ compiler was extended to experiment with new features, at a time when the C++ language was still evolving. Now that the C++ standard is complete, some of those features are superseded by superior alternatives. Using the old features might cause a warning in some cases that the feature will be dropped in the future. In other cases, the feature might be gone already.
While the list below is not exhaustive, it documents some of the options that are now deprecated:
-fexternal-templates-falt-external-templatesThese are two of the many ways for G++ to implement template instantiation. See Where’s the Template?. The C++ standard clearly defines how template definitions have to be organized across implementation units. G++ has an implicit instantiation mechanism that should work just fine for standard-conforming code.
-fstrict-prototype-fno-strict-prototypePreviously it was possible to use an empty prototype parameter list to indicate an unspecified number of parameters (like C), rather than no parameters, as C++ demands. This feature has been removed, except where it is required for backwards compatibility. See Backwards Compatibility.
G++ allows a virtual function returning ‘void *’ to be overridden by one returning a different pointer type. This extension to the covariant return type rules is now deprecated and will be removed from a future version.
The G++ minimum and maximum operators (‘<?’ and ‘>?’) and
their compound forms (‘<?=’) and ‘>?=’) have been deprecated
and are now removed from G++. Code using these operators should be
modified to use std::min and std::max instead.
The named return value extension has been deprecated, and is now removed from G++.
The use of initializer lists with new expressions has been deprecated, and is now removed from G++.
Floating and complex non-type template parameters have been deprecated, and are now removed from G++.
The implicit typename extension has been deprecated and is now removed from G++.
The use of default arguments in function pointers, function typedefs and other places where they are not permitted by the standard is deprecated and will be removed from a future version of G++.
G++ allows floating-point literals to appear in integral constant expressions, e.g. ‘ enum E { e = int(2.2 * 3.7) } ’ This extension is deprecated and will be removed from a future version.
G++ allows static data members of const floating-point type to be declared with an initializer in a class definition. The standard only allows initializers for static members of const integral types and const enumeration types so this extension has been deprecated and will be removed from a future version.