g++ main.cpp -o main: compile and run a C++ program

g++ main.cpp -o main compiles a C++ source file into an executable named main; ./main runs it. Use g++ (or clang++), not gcc, so the C++ standard library is linked. Add -std=c++20 to choose the language standard, -Wall -Wextra for warnings and -O2 for an optimized build.

Compile and run

The example uses Clang 20 from the ziglang package on PyPI (Zig ships a complete Clang with libc++), run through uv; cxx below stands for it. On your machine, write g++ or clang++:

cxx() { uv run -q --no-project --python 3.13 --with ziglang==0.15.1 python -m ziglang c++ -fno-color-diagnostics "$@"; }
cat > main.cpp <<'EOF'
#include <iostream>
#include <string>
#include <vector>
#include <numeric>

int main(int argc, char* argv[]) {
    std::vector<std::string> args(argv + 1, argv + argc);
    std::vector<int> nums{1, 2, 3, 4};
    int total = std::accumulate(nums.begin(), nums.end(), 0);
    std::cout << "sum = " << total << ", args = " << args.size() << '\n';
    for (const auto& a : args) std::cout << "  " << a << '\n';
    std::cout << "__cplusplus = " << __cplusplus << '\n';
}
EOF
cxx -std=c++20 -Wall -Wextra -O2 main.cpp -o main
./main
./main hello cpp

sum = 10, args = 0
__cplusplus = 202002
sum = 10, args = 2
  hello
  cpp
__cplusplus = 202002

__cplusplus shows the standard the file was compiled with: 202002 for C++20. main without a return statement returns 0 in C++.

Common options

Errors you will meet

• Compile errors name the file and line: a missing declaration gives was not declared in this scope (GCC) or use of undeclared identifier (Clang); a missing standard header no member named ... in namespace std; a wrong argument list no matching function for call to.
• Link errors appear after compilation succeeded: undefined reference to means a function was declared but its definition (another .cpp file or a library) was not linked.
• Standard too old. Features such as std::format (C++20) or structured bindings (C++17) fail on an older default; set the standard with -std.

Pitfalls

• gcc instead of g++. gcc main.cpp compiles but fails to link with undefined references to std:: symbols.
• Windows shells. In cmd.exe or PowerShell, run main.exe or .\main.exe; ./main is for Unix shells and Git Bash.
• MSVC. Visual Studio's compiler uses different options (cl /std:c++20 /EHsc main.cpp); the commands on this page are for GCC and Clang.

Confirm it worked

./main prints the expected output, and g++ --version shows which compiler built it. Recompile with -std=c++17 and check __cplusplus (201703) to see the standard switch in effect.

FAQ