It seems like in Ubuntu 18.04, the default shipped g++ (version 7.3.0) runs in C++11 compatible mode by default. I am getting some errors in my old codes which are not C++11 compatible. I installed g++-6 (version 6.4.0) and the programs are compiling fine. Is it possible to disable C++11 mode in g++-7?
man g++ you can select between different dialects:
Options Controlling C Dialect The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and Objective-C++) that the compiler accepts: -ansi In C mode, this is equivalent to -std=c90. In C++ mode, it is equivalent to -std=c++98. This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or of standard C++ (when compiling C++ code), such as the "asm" and "typeof" keywords, and predefined macros such as "unix" and "vax" that identify the type of system you are using. It also enables the undesirable and rarely used ISO trigraph feature. For the C compiler, it disables recognition of C++ style // comments as well as the "inline" keyword. -std= Determine the language standard. This option is currently only supported when compiling C or C++. The compiler can accept several base standards, such as c90 or c++98, and GNU dialects of those standards, such as gnu90 or gnu++98. When a base standard is specified, the compiler accepts all programs following that standard plus those using GNU extensions that do not contradict it. For example, -std=c90 turns off certain features of GCC that are incompatible with ISO C90, such as the "asm" and "typeof" keywords, but not other GNU extensions that do not have a meaning in ISO C90, such as omitting the middle term of a "?:" expression. On the other hand, when a GNU dialect of a standard is specified, all features supported by the compiler are enabled, even when those features change the meaning of the base standard. As a result, some strict-conforming programs may be rejected. The particular standard is used by -Wpedantic to identify which features are GNU extensions given that version of the standard. For example -std=gnu90 -Wpedantic warns about C++ style // comments, while -std=gnu99 -Wpedantic does not. A value for this option must be provided; possible values are c90 c89 iso9899:1990 Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as -ansi for C code. iso9899:199409 ISO C90 as modified in amendment 1. c99 c9x iso9899:1999 iso9899:199x ISO C99. This standard is substantially completely supported, modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G). See <http://gcc.gnu.org/c99status.html> for more information. The names c9x and iso9899:199x are deprecated. c11 c1x iso9899:2011 ISO C11, the 2011 revision of the ISO C standard. This standard is substantially completely supported, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The name c1x is deprecated. gnu90 gnu89 GNU dialect of ISO C90 (including some C99 features). gnu99 gnu9x GNU dialect of ISO C99. The name gnu9x is deprecated. gnu11 gnu1x GNU dialect of ISO C11. This is the default for C code. The name gnu1x is deprecated. c++98 c++03 The 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as -ansi for C++ code. gnu++98 gnu++03 GNU dialect of -std=c++98. c++11 c++0x The 2011 ISO C++ standard plus amendments. The name c++0x is deprecated. gnu++11 gnu++0x GNU dialect of -std=c++11. The name gnu++0x is deprecated. c++14 c++1y The 2014 ISO C++ standard plus amendments. The name c++1y is deprecated. gnu++14 gnu++1y GNU dialect of -std=c++14. This is the default for C++ code. The name gnu++1y is deprecated. c++1z The next revision of the ISO C++ standard, tentatively planned for 2017. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. gnu++1z GNU dialect of -std=c++1z. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.