CODEWITHSUNDEEP
c++stlnamespacesstdargument-dependent-lookup
Daniel Näslund
Daniel Näslund

Reputation: 2420

How avoid std naming conflicts due to Koenig lookup

As a learning exercise I've been reimplementing some of the STL algorithm. Even though I've not added any using directives or using declarations for the std namespace my test code won't compile unless I explicitly prefix those functions shadowing std names.

I assume that this is due to argument dependent lookup bringing in functions from the std namespace when I pass std::vector iterators as parameters to my functions.

A small program to illustrate my problem:

#include <vector>
#include <algorithm>

namespace danstd {
template <typename I, typename T>
I find(I b, I e, T val) {
    for (; b != e; ++b) {
        if (*b == val)
            return b;
    }
    return e;
}   
}

using namespace danstd;

int main() {
    std::vector<int> v = {1, 2, 3};

    auto i = find(begin(v), end(v), 3);
    return i == end(v) ? -1 : *i;
}

When I compile, I see these error messages:

$ g++ -Wall foo.cpp
foo.cpp: In function ‘int main()’:
foo.cpp:16:37: error: call of overloaded ‘find(std::vector<int>::iterator, std::vector<int>::iterator, int)’ is ambiguous
     return *find(begin(v), end(v), 3);
                                     ^
foo.cpp:5:3: note: candidate: I find(I, I, T) [with I = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; T = int]
 I find(I b, I e, T val) {
   ^~~~
In file included from /usr/include/c++/6/algorithm:62:0,
                 from foo.cpp:2:
/usr/include/c++/6/bits/stl_algo.h:3784:5: note: candidate: _IIter std::find(_IIter, _IIter, const _Tp&)[with _IIter = __gnu_cxx::__normal_iterator<int*, std::vector<int> >; _Tp = int]
     find(_InputIterator __first, _InputIterator __last,
     ^~~~

In the unit test code, I've linked to above, I've wrapped my functions inside a danstd namespace and I make each call on the form danstd::function(...). Is there any way around having to use fully qualified names to avoid the naming conflicts with std names?

Upvotes: 4

Views: 220

Answers (4)

Remy Lebeau
Remy Lebeau

Reputation: 598309

I make each call on the form danstd::function(...).

No, you are not. You have a using namespace danstd statement to dump your find() function into the global namespace, and then you are calling it without qualifying it.

So, either qualify it from the global namespace:

namespace danstd {
    ...
    template <typename I, typename T>
    I find(I b, I e, const T &val) { ... }
}

using namespace danstd;

auto i = ::find(begin(v), end(v), 3);

Or else drop the using statement and fully qualify the function call using your namespace:

namespace danstd {
    ...
    template <typename I, typename T>
    I find(I b, I e, const T &val) { ... }
}

// using namespace danstd;

auto i = danstd::find(begin(v), end(v), 3);

Upvotes: 1

Potatoswatter
Potatoswatter

Reputation: 137940

C++ is designed to associate functions with types in a given namespace. This is a valid assumption as long as namespaces are used to encapsulate libraries.

The types for your algorithms library are defined in the namespace of another library which already defines the same algorithms. One solution would be to implement your own types… Even one standard container is quite an exercise ;) .

Upvotes: 1

NathanOliver
NathanOliver

Reputation: 181068

I assume that this is due to argument dependent lookup bringing in functions from the std namespace when I pass std::vector iterators as parameters to my functions.

That is correct. Since std::vector::iterator lives in std it will do a lookup in std.

Is there any way around having to use fully qualified names to avoid the naming conflicts with std names?

Unfortunately no. You'll need to qualify you want the one from the global space like

return *::find(begin(v), end(v), 3);

Upvotes: 3

Cory Kramer
Cory Kramer

Reputation: 118021

You can disambiguate the call as follows

return *::find(begin(v), end(v), 3);

Without that, the compiler is correct, the call is ambiguous as you've noted due to argument dependent lookup

Upvotes: 2

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