Switch default c++ library from std=c++14 to std=c++17 on ubuntu

Question

I have tried to install a package on ubuntu that needs c++17 or newer libraries. I installed gcc-10 and g++-10. I also found that the default c++ library is c++14 by using this code:

man g++ | grep "This is the default for C++ code"

But I don't know how to change it to other versions. To run a simple code we can use -std=c++17. But I think Installing a package needs to change the default library.

*"package on ubuntu that needs c++17 or newer"* How did you determine this? — HolyBlackCat, Sep 18 '21 at 07:20
@HolyBlackCat the compiler gives an error that "string_view" works in c++17 ongoing — Mohammad Mohammadidoust, Sep 18 '21 at 08:17
@n.1.8e9-where's-my-sharem. at first it was gcc-9 but I changed it to gcc-10 by using update-alternatives — Mohammad Mohammadidoust, Sep 18 '21 at 08:19
gcc-9 and the library that comes with it support C++17. You don't need to install gcc-10 (but you may if you wish). You need to pass `-std=c++17` command line parameter to g++ each time you compile because it is not the default neither in gcc-9 nor in gcc-10 (c++17 is the default starting from gcc-11). — n. m. could be an AI, Sep 18 '21 at 08:46
Switching between language versions (`-std=c++17`, etc) doesn't involve switching between different standard library versions. Each GCC version comes with its own C++ standard library, and I don't think it can be changed. — HolyBlackCat, Sep 18 '21 at 09:15
How did you try to compile it? It depends on the build system, how to specify additional options to the invocation of compiler. — enedil, Sep 18 '21 at 09:24
@enedil the package installs via a make file. there is no option to set standard library version. do you mean I should change the make file? — Mohammad Mohammadidoust, Sep 18 '21 at 09:30
Try `make CXXFLAGS=-std=c++17`, you don't need to edit the makefile. — enedil, Sep 18 '21 at 09:31
If you need to compile it, it's not a package (yet). It's just a library. — HolyBlackCat, Sep 18 '21 at 11:19
"I have tried to install a package... but...". What was the compilation error, then, if any? — zkoza, Sep 25 '21 at 12:30

score 0 · Accepted Answer · answered Sep 18 '21 at 10:22

The c++ standard library traditionally is part of the compiler. On GNU/Linux systems typically GCC is used in conjunction with its standard library. An alternative would be CLang.

Note that the standard version is not only about the library, but even more so about language features that need to be implemented by the compiler directly.

Each compiler version has a default standards version, but supports several. The correct way to select a specific standards version during compilation is through a compiler flag. This should be part of a software's build system if the software depends on a specific version.

Compiler flags may be set in environmental variables, e.g. CXXFLAGS. These are then recognized by the popular build helpers like GNU Make or CMake. As the standards version to use is closely tied to the piece of software to be compiled, it would not be good practice to put the standards version into the environment.

But is it a problem if several pieces of software are built with a different standards version? No, not at all. Even when you link them together, the ABI compatibility is given across standards. See https://stackoverflow.com/a/49119902/21974 for a detailed answer on that topic.

Note that packages you install in Ubuntu are all pre-compiled, so you have no sway over how they are compiled.

Switch default c++ library from std=c++14 to std=c++17 on ubuntu

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