Colorful output on terminal

I went from manually outputting escape codes in each string to creating a utility struct/class full of static functions and variables that needed to be initialized in source files (static private members are required to) which was a bit ugly for me. Then I was searching for something different and while playing with enums, I found it interesting for my scenario. And then I carefully used namespaces to construct my sort-of function only header and here it is. Also, I'm not supporting pre-C++11 version of compilers.

My aims were – not to use classes since they became ugly with lots of static members and creating an object and using it for colouring doesn't looks so good (however I might try that in future), and to make it fast and also modern C++ish. Along the way, some neat tricks with C++ namespaces made it possible to sort of hide (well at least to code completions) my non-interface elements and give a nice shape to the code.

^{*I also want to reduce some not so necessary headers (e.g. <cstring>) if possible.}

#ifndef RANG_H
#define RANG_H

#include <iostream>
#include <cstdlib>
#include <cstring>
extern "C" {
#include <unistd.h>
}

namespace rang {

    enum class style : unsigned char {
        Reset     = 0,
        bold      = 1,
        dim       = 2,
        italic    = 3,
        underline = 4,
        blink     = 5,
        reversed  = 6,
        conceal   = 7,
        crossed   = 8
    };
    enum class fg : unsigned char {
        def     = 39,
        black   = 30,
        red     = 31,
        green   = 32,
        yellow  = 33,
        blue    = 34,
        magenta = 35,
        cyan    = 36,
        gray    = 37
    };
    enum class bg : unsigned char {
        def     = 49,
        black   = 40,
        red     = 41,
        green   = 42,
        yellow  = 43,
        blue    = 44,
        magenta = 45,
        cyan    = 46,
        gray    = 47
    };
}


namespace {
    bool isAllowed = false;
    bool isTerminal()
    {
        return isatty(STDOUT_FILENO);
    }
    bool supportsColor()
    {
        if(const char *env_p = std::getenv("TERM")) {
            const char *const term[8] = {
                "xterm", "xterm-256", "xterm-256color", "vt100",
                "color", "ansi",      "cygwin",         "linux"};
            for(unsigned int i = 0; i < 8; ++i) {
                if(std::strcmp(env_p, term[i]) == 0) return true;
            }
        }
        return false;
    }
    std::ostream &operator<<(std::ostream &os, rang::style v)
    {
        return isAllowed ? os << "\e[" << static_cast<int>(v) << "m" : os;
    }
    std::ostream &operator<<(std::ostream &os, rang::fg v)
    {
        return isAllowed ? os << "\e[" << static_cast<int>(v) << "m" : os;
    }
    std::ostream &operator<<(std::ostream &os, rang::bg v)
    {
        return isAllowed ? os << "\e[" << static_cast<int>(v) << "m" : os;
    }
    namespace init {
        void rang()
        {
            isAllowed = isTerminal() && supportsColor() ? true : false;
        }
    }
}
#endif /* ifndef RANG_H*/

And to test:

#include <iostream>
#include "rang.h"

int main()
{
    init::rang();
    std::cout << rang::style::bold << rang::fg::red << "ERROR HERE! "
              << std::endl
              << rang::bg::red << rang::fg::gray << "ERROR INVERSE?"
              << rang::style::Reset << std::endl;
    return 0;
}

And here are warnings if you don't want to compile:

agauniyal@lenovo > rang[master] » clang++ test.cpp -std=c++11 -o rang.app -Wall -Weverything
In file included from test.cpp:2:
./rang.h:13:7: warning: scoped enumerations are incompatible with C++98 [-Wc++98-compat]
        enum class style : unsigned char {
             ^
./rang.h:13:2: warning: enumeration types with a fixed underlying type are incompatible with C++98 [-Wc++98-compat]
        enum class style : unsigned char {
        ^
./rang.h:24:7: warning: scoped enumerations are incompatible with C++98 [-Wc++98-compat]
        enum class fg : unsigned char {
             ^
./rang.h:24:2: warning: enumeration types with a fixed underlying type are incompatible with C++98 [-Wc++98-compat]
        enum class fg : unsigned char {
        ^
./rang.h:35:7: warning: scoped enumerations are incompatible with C++98 [-Wc++98-compat]
        enum class bg : unsigned char {
             ^
./rang.h:35:2: warning: enumeration types with a fixed underlying type are incompatible with C++98 [-Wc++98-compat]
        enum class bg : unsigned char {
        ^
./rang.h:69:29: warning: use of non-standard escape character '\e' [-Wpedantic]
                return isAllowed ? os << "\e[" << static_cast<int>(v) << "m" : os;
                                          ^~
./rang.h:73:29: warning: use of non-standard escape character '\e' [-Wpedantic]
                return isAllowed ? os << "\e[" << static_cast<int>(v) << "m" : os;
                                          ^~
./rang.h:77:29: warning: use of non-standard escape character '\e' [-Wpedantic]
                return isAllowed ? os << "\e[" << static_cast<int>(v) << "m" : os;
                                          ^~
./rang.h:80:8: warning: no previous prototype for function 'rang' [-Wmissing-prototypes]
                void rang()
                     ^
test.cpp:7:21: warning: enumeration type in nested name specifier is incompatible with C++98 [-Wc++98-compat]
        std::cout << rang::style::bold << rang::fg::red << "ERROR HERE! "
                           ^
test.cpp:7:42: warning: enumeration type in nested name specifier is incompatible with C++98 [-Wc++98-compat]
        std::cout << rang::style::bold << rang::fg::red << "ERROR HERE! "
                                                ^
test.cpp:9:21: warning: enumeration type in nested name specifier is incompatible with C++98 [-Wc++98-compat]
                  << rang::bg::red << rang::fg::gray << "ERROR INVERSE?"
                           ^
test.cpp:9:38: warning: enumeration type in nested name specifier is incompatible with C++98 [-Wc++98-compat]
                  << rang::bg::red << rang::fg::gray << "ERROR INVERSE?"
                                            ^
test.cpp:10:21: warning: enumeration type in nested name specifier is incompatible with C++98 [-Wc++98-compat]
                  << rang::style::Reset << std::endl;
                           ^
15 warnings generated.