Compile C++ with GCC: g++ commands and flags

To compile C++ with GCC, run g++ main.cpp -o main, then start the program with ./main (main.exe on Windows). Use the g++ command rather than gcc, because g++ links the C++ standard library for you. Add -std=c++20 to choose the language standard and -Wall -Wextra to have GCC warn about questionable code.

Save this as main.cpp:

#include <iostream>

int main() {
    std::cout << "Hello, GCC!\n";
}

Compile it, then run it:

g++ main.cpp -o main
./main

The program printed Hello, GCC!. The samples on this page are C++17 (one needs C++20), built with GCC 16.2 on Linux, through Compiler Explorer, and also with MSVC 19.44, Clang 19.1 (MSVC STL) and Clang 23.1 (libc++); the messages shown are GCC’s. Since When lists the compiler releases for each standard, and Toolchain lists GCC with the other compilers.

g++ or gcc: which command compiles C++?

Use g++. Both commands run GCC; for a .cpp file they differ only at the link, the step that joins your compiled code with the libraries it uses into one program. GCC’s manual says g++ “is a program that calls GCC and automatically specifies linking against the C++ library”.

gcc main.cpp still compiles the file as C++, because of its .cpp extension, but does not add that library, so the link fails. With GCC 16.2’s gcc, building the program above stopped at undefined reference to `std::cout' and ended with collect2: error: ld returned 1 exit status (ld is the linker). Adding -lstdc++, which links libstdc++, GCC’s C++ standard library, made it build; g++ does that for you.

How do you choose the C++ standard?

Pass -std= with the standard’s name: -std=c++17, -std=c++20 or -std=c++23. Without it, GCC uses a default that depends on the release: GCC 11 to 15 use gnu++17, and GCC 16 “changes the default language version for C++ compilation from -std=gnu++17 to -std=gnu++20” (gnu++20 is the GNU dialect of C++20). Name the standard yourself, so every machine builds the same language.

This program uses std::format, which C++20 added:

#include <format>
#include <iostream>
#include <string>

int main() {
    std::string name = "GCC";
    std::cout << std::format("Hello, {}!\n", name);  // C++20
}

With -std=c++17, GCC 16.2 refused it:

02-standard.cpp: In function 'int main()':
02-standard.cpp:8:23: error: 'format' is not a member of 'std'
    8 |     std::cout << std::format("Hello, {}!\n", name);  // C++20
      |                       ^~~~~~
02-standard.cpp:8:23: note: 'std::format' is only available from C++20 onwards

With -std=c++20 it built and printed Hello, GCC!. std::format also needs GCC 13 or newer.

Which GCC flags does a beginner need?

These ten GCC compiler flags, also called options or switches, are the ones a beginner meets first. What each does, in the terms of GCC’s manual:

Warnings point at mistakes that still compile. This program has two:

#include <iostream>
#include <string>
#include <vector>

int main() {
    std::vector<std::string> names{"Ada", "Linus"};
    int greeted = 0;  // declared, never used
    for (int i = 0; i < names.size(); ++i) {  // int against unsigned
        std::cout << "Hello, " << names[i] << "!\n";
    }
}

Built with -std=c++17 alone, GCC 16.2 printed nothing, and the program greeted both names. With -Wall -Wextra added, it printed:

03-warnings.cpp: In function 'int main()':
03-warnings.cpp:10:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::__cxx11::basic_string<char> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   10 |     for (int i = 0; i < names.size(); ++i) {  // int against unsigned
      |                     ~~^~~~~~~~~~~~~~
03-warnings.cpp:9:9: warning: unused variable 'greeted' [-Wunused-variable]
    9 |     int greeted = 0;  // declared, never used
      |         ^~~~~~~

The program still built and ran: a warning does not stop the build, so read each one.

How do you compile several .cpp files?

Name every source file in one command, and g++ compiles each one and links them into one program. In the running example, main.cpp calls greet(), which the header file greet.h declares and greet.cpp defines:

# compile and link in one command
g++ main.cpp greet.cpp -o app
# or in two steps: an object file per source, then the link
g++ -c main.cpp
g++ -c greet.cpp
g++ main.o greet.o -o app

The one command built a program that printed Hello from greet.cpp!. The two steps follow GCC’s manual: -c stops before the link and leaves an object file (.o), one source file’s compiled code, and the link “combines all the object files” into the program. What a compiler does explains object files and the linker.

Leave a file out and the link fails. Built without greet.cpp, GCC 16.2 printed this after a line with the linker’s path (example.cpp is Compiler Explorer’s name for main.cpp):

example.cpp:(.text+0x5): undefined reference to `greet()'
collect2: error: ld returned 1 exit status

The header declared greet(), so main.cpp compiled, but no linked file defined it. For more than a handful of files, use a build system such as CMake instead of typing the commands.

How do you install GCC?

On Windows, install GCC through MSYS2, and on Ubuntu with apt-get; then check it with g++ --version. These commands come from VS Code’s setup guides:

# Windows: run the MSYS2 installer, then, in MSYS2
pacman -S --needed base-devel mingw-w64-ucrt-x86_64-toolchain
# Ubuntu: update the package lists, then install
sudo apt-get update
sudo apt-get install build-essential gdb
# macOS: Apple's command line developer tools
xcode-select --install
# then, on any of them
g++ --version

• Windows: add C:\msys64\ucrt64\bin, the default install’s folder, to PATH, then run g++ --version in a new Command Prompt (Using GCC with MinGW).
• Ubuntu: the second command installs the compiler and the GDB debugger (Using C++ on Linux in VS Code).
• macOS: Apple’s tools “include Clang”, in the words of VS Code’s macOS guide. g++ --version shows which compiler your g++ runs, and the messages on this page came from GCC 16.2; Clang vs GCC covers where the two compilers differ.

What do the common GCC errors mean?

Most first errors are one of three, and each comes from a different step: the link, the compile, or the shell before GCC runs. The link and compile messages are GCC’s; command not found and is not recognized are the shell’s:

The commands at a glance

Tooling has more on compilers, build systems and the tools around them.