Which C++ standard is the default when compiling with g++?

Question

I have a piece of code that looks like the following. Let's say it's in a file named example.cpp

#include <fstream> #include <string> // line added after edit for clarity  int main() {     std::string filename = "input.txt";     std::ifstream in(filename);      return 0; } 

On a windows, if I type in the cmd the command g++ example.cpp, it will fail. It's a long list of errors I think mostly due to the linker complaining about not being able to convert from string to const char*.

But if I run the compiler using an additional argument like so: g++ -std=c++17 example.cpp, it will compile and work fine with no problems.

What happens when I run the former command? I'm guessing a default version standard of the C++ compiler gets called, but I don't know which? And as a programmer/developer, should I always use the latter command with the extra argument?

Answer 1

If your version of g++ is later than 4.7 I think you can find the default version of C++ standard supported like so:

g++ -dM -E -x c++  /dev/null | grep -F __cplusplus 

An example from my machine:

mburr@mint17 ~ $ g++ --version | head -1 g++ (Ubuntu 4.8.4-2ubuntu1~14.04.3) 4.8.4 mburr@mint17 ~ $ g++ -dM -E -x c++  /dev/null | grep -F __cplusplus #define __cplusplus 199711L 

Some references:

• Details on the g++ options used
• Why this only works for g++ 4.7 or later

Answer 2

You can also check with gdb

1. $ g++ example.cpp -g Compile program with -g flag to generate debug info
2. $ gdb a.out Debug program with gdb
3. (gdb) b main Put a breakpoint at main
4. (gdb) run Run program (will pause at breakpoint)
5. (gdb) info source

Prints out something like:

Current source file is example.cpp Compilation directory is /home/xxx/cpp Located in /home/xxx/cpp/example.cpp Contains 7 lines. Source language is c++. Producer is GNU C++14 6.3.0 20170516 -mtune=generic -march=x86-64 -g. Compiled with DWARF 2 debugging format. Does not include preprocessor macro info. 

There is the standard used by compiler: Producer is GNU C++14

If you recompile your program using -std=c++11 (for example), gdb detects it: Producer is GNU C++11