g++ -pedantic-errors and -Wpedantic: reject non-standard C++ extensions

g++ -std=c++17 -pedantic-errors main.cpp makes the compiler reject code that the ISO C++ standard does not allow but GCC or Clang accept as an extension: variable-length arrays, designated initializers before C++20, zero-size arrays, __int128 and similar. -Wpedantic (also spelled -pedantic) reports the same issues as warnings. Use them to keep code portable to MSVC and other compilers.

Extensions accepted by default

The example uses Clang 20 from the Zig 0.15.1 toolchain (installed from PyPI with uv); cxx below is that compiler, and g++ accepts the same options:

cxx() { uv run -q --no-project --python 3.13 --with ziglang==0.15.1 python -m ziglang c++ -fno-color-diagnostics "$@"; }
cat > ext.cpp <<'EOF'
#include <cstdio>
int main(int argc, char**) {
    int n = argc + 2;
    int vla[n];                           // variable-length array: not C++
    vla[0] = 1;
    struct P { int x, y; } p = { .x = 1, .y = 2 };   // designated initializers: C++20
    std::printf("%d %d\n", vla[0], p.y);
}
EOF
show() { cxx "$@" ext.cpp -o ext 2>&1 | grep -E '(warning|error):'; echo "exit: ${PIPESTATUS[0]}"; }
echo "== -std=c++17"
show -std=c++17
echo "== -std=c++17 -Wpedantic"
show -std=c++17 -Wpedantic
echo "== -std=c++17 -pedantic-errors"
show -std=c++17 -pedantic-errors
echo "== -std=c++20 -pedantic-errors"
show -std=c++20 -pedantic-errors

== -std=c++17
ext.cpp:4:13: warning: variable length arrays in C++ are a Clang extension [-Wvla-cxx-extension]
exit: 0
== -std=c++17 -Wpedantic
ext.cpp:4:13: warning: variable length arrays in C++ are a Clang extension [-Wvla-cxx-extension]
ext.cpp:6:34: warning: designated initializers are a C++20 extension [-Wc++20-designator]
exit: 0
== -std=c++17 -pedantic-errors
ext.cpp:4:13: error: variable length arrays in C++ are a Clang extension [-Werror,-Wvla-cxx-extension]
ext.cpp:6:34: error: designated initializers are a C++20 extension [-Werror,-Wc++20-designator]
exit: 1
== -std=c++20 -pedantic-errors
ext.cpp:4:13: error: variable length arrays in C++ are a Clang extension [-Werror,-Wvla-cxx-extension]
exit: 1

Plain -std=c++17 builds the program and warns only about the VLA (Clang has that warning on by default; GCC accepts VLAs silently). -Wpedantic adds the designated-initializer warning; -pedantic-errors turns both into errors and the build fails. With -std=c++20 designated initializers are standard, so only the VLA remains.

The three levels

Common extensions it catches

Pitfalls

• -pedantic-errors follows the -std level: always pass an explicit -std=c++17 (or newer, see -std=c++17), otherwise the compiler's default dialect decides what counts as an extension.
• System and third-party headers can trigger pedantic diagnostics; include them with -isystem so diagnostics in those headers are suppressed.
• It is not a general warning switch: combine with -Wall -Wextra for bug-finding warnings.
• -pedantic does not detect every non-portable construct, only those for which the standard requires a diagnostic; undefined behaviour is not reported.

FAQ