One-liner for nested for loops in C++

Ranges are not avalible, but range based loops come quite close.

#include <iostream>
int main(){
    for (int i:{1,2,3}) { for (int j:{1,2,3}) {std::cout << i << " " << j <<std::endl;}};
}

or if you like to use the same range

#include <iostream>
int main(){
    const auto range {1,2,3};
    for (int i:range) {for (int j:range) {std::cout << i << " " << j <<std::endl;}};
}

and just for the fun of it with std::for_each (this one is perhaps hard to read, but it has no hand written loops)

#include <iostream>
#include <algorithm>
int main(){
    const auto range {1,2,3};
    std::for_each(range.begin(), range.end(), [range](int i) {std::for_each(range.begin(), range.end(), [i](int j) {std::cout << i << " " << j <<std::endl; } ); } );
}

Looping example (updated):

#include <array>
#include <iostream>
#include <utility>

template<int VRange, int VRepCount, int VValueRIndex = VRepCount> class
t_Looper
{
    public: template<typename TAction> static void
    process(::std::array<int, VRepCount> & values, TAction && action)
    {
        for(;;)
        {
            t_Looper<VRange, VRepCount, VValueRIndex - 1>::process(values, ::std::forward<TAction>(action));
            auto & value{values[VRepCount - VValueRIndex]};
            if((VRange - 1) != value)
            {
                ++value;
            }
            else
            {
                value = 0;
                break;
            }
        }
    }
};

template<int VRange, int VRepCount> class
t_Looper<VRange, VRepCount, 0>
{
    private: template<int... VIndexes, typename TAction> static void
    invoke(::std::integer_sequence<int, VIndexes...>, ::std::array<int, VRepCount> const & values, TAction && action)
    {
        action(values[VIndexes]...);
    }

    public: template<typename TAction> static void
    process(::std::array<int, VRepCount> & values, TAction && action)
    {
        invoke(::std::make_integer_sequence<int, VRepCount>(), values, ::std::forward<TAction>(action));
    }
};

template<int VRange, int VRepCount, typename TAction> void
multiloop(TAction && action)
{
    ::std::array<int, VRepCount> values{};
    t_Looper<VRange, VRepCount>::process(values, ::std::forward<TAction>(action));
}

int main()
{
    multiloop<3, 2>([](int i, int j){::std::cout << i << " " << j << ::std::endl;});
    multiloop<3, 4>([](int i, int j, int k, int l){::std::cout << i << " " << j << " " << k << " " << l << ::std::endl;});
    return(0);
}

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Is it possible to have an easy-to-read, non-boost version of this in C++?

No.

You cannot do that in pure C++. You would need a library or so.

There is an Extension for ranges that is experimental in C++14, but even with this, I am not sure if could make it.

if you do not mind creating your own .. dot-dot operator with the help of these two template functions:

template< int First, int Last , int Step = 1>
int ( &dotdot() )[ ( Step + Last - First ) /  Step ]
{
    static int result[ ( Step + Last - First ) /  Step ];

    for( int index = First; index <= Last; index += Step ){
        result[ ( index - First ) / Step ] = index;
    }

    return result;
}

template< int Last, int First, int Step = 1 >
int ( &dotdot() )[ ( Step + Last - First ) / Step ]
{
    static int result[ ( Step + Last - First ) / Step ];

    for( int index = Last; index >= First; index -= Step ){
        result[ ( Last - index ) / Step ] = index;
    }

    return result;
}

then you can:

for( int i : dotdot<0,2>() ) for( int j : dotdot<0,2>() ) std::cout << i << ' ' << j << '\n';

and the output:

0 0
0 1
0 2
1 0
1 1
1 2
2 0
2 1
2 2

usage:

• dotdot<'a','z'>() returns a to z
• dotdot<'z','a',2>() returns z to a and step is 2
• dotdot<-10,0>() returns -10 to 0
• dotdot<-10,10,3>() returns -10 to 10 and step is 3