Compiler Flags

gcc vs g++

The main differences:

• gcc will compile: .c/.cpp files as C and C++ respectively.

• g++ will compile: .c/.cpp files but they will all be treated as C++ files.

• Also if you use g++ to link the object files it automatically links in the std C++ libraries (gcc does not do this).

Compiling

$ clang++-6.0 main.cpp

Searching through clang documentation

clang --help | grep "\-I "

$ clang --help | grep "\-o "
  -o <file>               Write output to <file>

Flags

• --version

• -c

• -std=<standard>

• -o <file>

• -D<macroname>=<value>

• -I<directory>

• -shared, --shared

--version

$ clang++-6.0 --version
clang version 6.0.0-1ubuntu2

-c , --compile

Only run preprocess, compile, and assemble steps no linking generates a target “.o” object file.

$ clang++-6.0 -c main.cpp
$ ls
main.cpp main.o

-std=<standard>

sets standard library. This must be set if you are to use new features of c++. Use -std=c++2a for c++20

$ clang++-6.0 main.cpp -std=c++14

-o <file>

The default name of the output file is a.out but this can be changed

$ clang++-6.0 main.cpp -o app.out
$ ls
app.out main.cpp

-D<macroname>=<value>

The macro flag

If we have the following file: main.cpp

#include <iostream>

void main(){
    #ifdef DEBUG   
       std::cout << "Debug run\n";
    #else
       std::cout << "Release run\n";
    #endif
}

We can choose which code to run using -D

$ clang++-6.0 -D DEBUG main.cpp -o app.out
$ ./app.out
Debug run

or this without a space

$ clang++-6.0 -DDEBUG main.cpp -o app.out

or running this:

$ clang++-6.0 main.cpp -o app.out
$ ./app.out
Release run

-I<dir>, --include-directory <arg>, --include-directory=<arg>

If we had these 3 files but MyClass.cpp and MyClass.hpp were in a different folder

include/MyClass.hpp

#pragma once
#include <iostream>
struct MyClass {
    static void print();
};

src/MyClass.cpp

#include "MyClass.hpp"
void MyClass::print(){
    std::cout << "print MyClass\n";
}

src/main.cpp

#include "MyClass.hpp"

int main(){
    MyClass::print();
    return 0;
}

To compile we would have to do something like this:

clang++-6.0 main.cpp MyClass.cpp -I../include

This will look for our headers in the include directory. This allows us to organize our files.

A few more important flags

• -shared, --shared

• --cuda-path=

g++/gcc specific flag

In order to compile with all warnings enabled and to produce standards-compatible C++ code, I recommend using the flags

-Wall -ansi

more info clang more info 2nd link