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Enum and Namespace mystery

sansoftaus (16)
Hi Friends,

Am trying to work with enum + namespaces combined, I landed into some confusing problems, your clear explanation will help me understand this concept very well.

a) My namespaces header file:
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//namespaces.h

namespace san{

    void say_hi(){
        std::cout<<"Hi there !"<<std::endl ;
    }
    typedef enum{
        Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday
    } Week ;
}


b) My main program file:
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//First_go.cpp

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

int main(int argC, char *argV[]){

    san::say_hi() ;
    san::Week Today ;
    Today = san::Week::Monday ;
    std::cout<<"Today = "<<Today<<std::endl ;

    return 0 ;
}


c) Output Error:
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g++ -o First_go ./First_go.cpp
./First_go.cpp: In function ‘int main(int, char**)’:
./First_go.cpp:8:18: error: ‘san::Week’ is not a class or namespace
Today = san::Week::Monday ;


Am just expecting it to print 0, since Monday is 0 right ??
Many thanks in advance :)

Cheers,
San
LB (13399)
C-style Unscoped enums place their elements into the enclosing namespace. C++ scoped enums create their own namespace.

http://en.cppreference.com/w/cpp/language/enum#Scoped_enumerations.28since_C.2B.2B11.29
Last edited on
sansoftaus (16)
Hi thanks, So I cannot place enum inside a namespace ? san::Week::Monday doesn't work...

JLBorges (13770)
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namespace san{

    enum class week { // scoped enum
        Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday
    };

    enum colour { // unscoped enum
        Red, Green, Blue, Black, Grey, White
    };
}

int main()
{
    san::week today = san::week::Monday ; // scoped enum

    san::colour cloudy = san::Grey ; // unscoped enum
}


Note: A C++ unscoped enum is not a C-style enum. There is no C-style enum in C++.
LB (13399)
Sorry, I used the term "C-style" because their behavior comes from C. I won't use that term to refer to unscoped enums again.
sansoftaus (16)
Awesome, thanks it worked with -std=c++11 option but cout dint work:

std::cout<<"Today = "<<Today<<std::endl ;

even I tried to cast it:

std::cout<<"Today = "<<static_cast<std::string>(Today)<<std::endl ;

Error:

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g++ -o First_go ./First_go.cpp -std=c++11
./First_go.cpp: In function ‘int main(int, char**)’:
./First_go.cpp:10:58: error: no matching function for call to ‘std::basic_string<char>::basic_string(san::Week&)’
std::cout<<"Today = "<<static_cast<std::string>(Today)<<std::endl ;
^
./First_go.cpp:10:58: note: candidates are:
In file included from /usr/include/c++/4.8/string:52:0,
from /usr/include/c++/4.8/bits/locale_classes.h:40,
from /usr/include/c++/4.8/bits/ios_base.h:41,
from /usr/include/c++/4.8/ios:42,
from /usr/include/c++/4.8/ostream:38,
from /usr/include/c++/4.8/iostream:39,
from ./First_go.cpp:3:
/usr/include/c++/4.8/bits/basic_string.h:532:9: note: template<class _InputIterator> std::basic_string<_CharT, _Traits, _Alloc>::basic_string(_InputIterator, _InputIterator, const _Alloc&)
basic_string(_InputIterator __beg, _InputIterator __end,
^
/usr/include/c++/4.8/bits/basic_string.h:532:9: note: template argument deduction/substitution failed:
./First_go.cpp:10:58: note: candidate expects 3 arguments, 1 provided
std::cout<<"Today = "<<static_cast<std::string>(Today)<<std::endl ;
^
In file included from /usr/include/c++/4.8/string:52:0,
from /usr/include/c++/4.8/bits/locale_classes.h:40,
from /usr/include/c++/4.8/bits/ios_base.h:41,
from /usr/include/c++/4.8/ios:42,
from /usr/include/c++/4.8/ostream:38,
from /usr/include/c++/4.8/iostream:39,
from ./First_go.cpp:3:
/usr/include/c++/4.8/bits/basic_string.h:522:7: note: std::basic_string<_CharT, _Traits, _Alloc>::basic_string(std::initializer_list<_Tp>, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char
>; _Alloc = std::allocator<char>]
basic_string(initializer_list<_CharT> __l, const _Alloc& __a = _Alloc());
^
/usr/include/c++/4.8/bits/basic_string.h:522:7: note: no known conversion for argument 1 from ‘san::Week’ to ‘std::initializer_list<char>’
/usr/include/c++/4.8/bits/basic_string.h:507:7: note: std::basic_string<_CharT, _Traits, _Alloc>::basic_string(std::basic_string<_CharT, _Traits, _Alloc>&&) [with _CharT = char; _Traits = std::char_traits<c
har>; _Alloc = std::allocator<char>]
basic_string(basic_string&& __str) noexcept
^
/usr/include/c++/4.8/bits/basic_string.h:507:7: note: no known conversion for argument 1 from ‘san::Week’ to ‘std::basic_string<char>&&’
/usr/include/c++/4.8/bits/basic_string.h:497:7: note: std::basic_string<_CharT, _Traits, _Alloc>::basic_string(std::basic_string<_CharT, _Traits, _Alloc>::size_type, _CharT, const _Alloc&) [with _CharT = ch
ar; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; std::basic_string<_CharT, _Traits, _Alloc>::size_type = long unsigned int]
basic_string(size_type __n, _CharT __c, const _Alloc& __a = _Alloc());
^
/usr/include/c++/4.8/bits/basic_string.h:497:7: note: candidate expects 3 arguments, 1 provided
/usr/include/c++/4.8/bits/basic_string.h:490:7: note: std::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _CharT*, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = s
td::allocator<char>]
basic_string(const _CharT* __s, const _Alloc& __a = _Alloc());
^
/usr/include/c++/4.8/bits/basic_string.h:490:7: note: no known conversion for argument 1 from ‘san::Week’ to ‘const char*’
/usr/include/c++/4.8/bits/basic_string.h:483:7: note: std::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _CharT*, std::basic_string<_CharT, _Traits, _Alloc>::size_type, const _Alloc&) [with _Cha
rT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; std::basic_string<_CharT, _Traits, _Alloc>::size_type = long unsigned int]
basic_string(const _CharT* __s, size_type __n,
^
/usr/include/c++/4.8/bits/basic_string.h:483:7: note: candidate expects 3 arguments, 1 provided
/usr/include/c++/4.8/bits/basic_string.h:471:7: note: std::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::basic_string<_CharT, _Traits, _Alloc>&, std::basic_string<_CharT, _Traits, _Alloc>::
size_type, std::basic_string<_CharT, _Traits, _Alloc>::size_type, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; std::basic_string<_CharT, _Traits, _All
oc>::size_type = long unsigned int]
basic_string(const basic_string& __str, size_type __pos,
^
/usr/include/c++/4.8/bits/basic_string.h:471:7: note: candidate expects 4 arguments, 1 provided
/usr/include/c++/4.8/bits/basic_string.h:462:7: note: std::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::basic_string<_CharT, _Traits, _Alloc>&, std::basic_string<_CharT, _Traits, _Alloc>::
size_type, std::basic_string<_CharT, _Traits, _Alloc>::size_type) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; std::basic_string<_CharT, _Traits, _Alloc>::size_type
= long unsigned int]
basic_string(const basic_string& __str, size_type __pos,
^
/usr/include/c++/4.8/bits/basic_string.h:462:7: note: candidate expects 3 arguments, 1 provided
/usr/include/c++/4.8/bits/basic_string.h:455:7: note: std::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = char; _Traits = std::char_tra
its<char>; _Alloc = std::allocator<char>]
basic_string(const basic_string& __str);
^
/usr/include/c++/4.8/bits/basic_string.h:455:7: note: no known conversion for argument 1 from ‘san::Week’ to ‘const std::basic_string<char>&’
/usr/include/c++/4.8/bits/basic_string.h:448:7: note: std::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<c
har>]
basic_string(const _Alloc& __a);
^
/usr/include/c++/4.8/bits/basic_string.h:448:7: note: no known conversion for argument 1 from ‘san::Week’ to ‘const std::allocator<char>&’
/usr/include/c++/4.8/bits/basic_string.h:437:7: note: std::basic_string<_CharT, _Traits, _Alloc>::basic_string() [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
basic_string()
^
/usr/include/c++/4.8/bits/basic_string.h:437:7:
Last edited on
LB (13399)
Enums have never been convertible to strings in C++. You need to provide your own helper function for that.
JLBorges (13770)
The underlying type of an enum is an integral type.
An enum can be converted to its underlying integral type (explicitly for scoped enums).

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#include <iostream>

namespace san {

    enum class week { // scoped enum
        Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday
    };

    enum colour { // unscoped enum
        Red, Green, Blue, Black, Grey, White
    };
}

namespace san2 {

    enum class week { // scoped enum
        Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday
    };
    std::ostream& operator<< ( std::ostream& stm, week w )
    {
        static const char* const names[] = {
            "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday", "Undefined"
        };
        std::size_t pos = std::size_t(w) ;
        if( pos > std::size_t(week::Sunday) ) pos = std::size_t(week::Sunday) + 1 ;
        return stm << names[pos] ;
    }

    enum colour { // unscoped enum
        Red, Green, Blue, Black, Grey, White
    };
    std::ostream& operator<< ( std::ostream& stm, colour c )
    {
        static const char* const names[] = {
            "Red", "Green", "Blue", "Black", "Grey", "White", "Undefined"
        };

        std::size_t pos = c ;
        if( pos > White ) pos = White + 1 ;
        return stm << names[pos] ;
    }
}

int main()
{
    {
        san::week today = san::week::Tuesday ;
        std::cout << int(today) << '\n' ; // 1
        // explicit conversion to int

        san::colour cloudy = san::Grey ;
        std::cout << cloudy << '\n' ; // 4
        // implicit conversion (promotion) to int
    }
    {
        san2::week today = san2::week::Tuesday ;
        std::cout << today << '\n' ; // Tuesday

        san2::colour cloudy = san2::Grey ;
        std::cout << cloudy << '\n' ; // Grey
    }
}

http://coliru.stacked-crooked.com/a/2ec9e1445d2eb779
sansoftaus (16)
Omg amazing, thanks a million JLBorges... Gosh its so intimidating... Dono if I can be a good C++ Programmer like you
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