To use update-alternatives to manage your g++ versions, you must first ensure a newer compiler (like GCC 11) is installed, register both versions with the system, and then select the new default. Here is the step-by-step process to set it up and verify it with a test compilation.

GCC to Default C++ Standard Mapping

G++ Version	Default C++ Standard Flag	Base ISO Standard
G++ 16	-std=gnu++20	C++20
G++ 11 to 15	-std=gnu++17	C++17
G++ 6 to 10	-std=gnu++14	C++14
G++ 5 and older	-std=gnu++98	C++98 / C++03

Install g++

Step 1: Install a newer compiler and register both versions

Run the following commands to install g++-11 and add both your current g++ and the new g++-11 to the update-alternatives system.

# 1. Add the toolchain test repository
sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y

# 2. Update your package lists to pull in the new repository data
sudo apt update

# 3. Install GCC 11
sudo apt update && sudo apt install g++-11 -y

# 4. Register your current system g++ (typically pointing to g++-9 on Ubuntu 20.04)
# We assign it a priority of 50
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-9 50

# 5. Register the new g++-11 with a higher priority of 100
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-11 100

# 6. Register the new g++-16 with a higher priority of 150
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-16 150

(Note: If your original compiler is not version 9, replace /usr/bin/g++-9 above with your actual version, which you can find by running ls /usr/bin/g++*).

Step 2: Choose your default version

If you ever want to manually toggle back and forth between them later, run this command:

sudo update-alternatives --config g++

You will see a menu like this.

Type the selection number for g++-11 and press Enter:
text
  Selection    Path              Priority   Status
------------------------------------------------------------
* 0            /usr/bin/g++-11    100       auto mode
  1            /usr/bin/g++-9     50        manual mode
  2            /usr/bin/g++-11    100       manual mode

Press <enter> to keep the current choice[*], or type selection number:

Step 3: Verify by checking the macro and compiling

Now, let's verify that your system defaults to the new version and can natively compile C++20 without changing any flags.

Check the new default macro standard

Run your macro query again. It should now output 201703L (C++17) as the bare default:

echo | g++ -dM -E -x c++ - | grep __cplusplus

• 202002L indicates C++20
• 201703L indicates C++17
• 201402L indicates C++14
• 201103L indicates C++11

Test a C++20 compilation using the standard flag

Because g++-11 recognizes the finalized standard, your previous one-liner with -std=c++20 will now compile and execute flawlessly:

echo -e '#include <iostream>\nint main() {\n    std::cout << "Hello from C++20!" << std::endl;\n    return 0;\n}' > main.cpp && g++ -std=c++20 main.cpp -o program && ./program

Test a C++20 compilation using c++20 feature:

echo -e '#include <iostream>\n#include <concepts>\n\ntemplate<typename T>\nconcept Numeric = std::integral<T> || std::floating_point<T>;\n\nvoid print_val(Numeric auto x) {\n    std::cout << "Native C++20 Concept Works! Value: " << x << std::endl;\n}\n\nint main() {\n    print_val(2026);\n    return 0;\n}' > main.cpp && g++ main.cpp -o program && ./program