std::remove vs std::remove_if

Question

I have a following class:

class Foo
{
public:
    void Fill();
private:
    std::vector<std::wstring> vec;
};

And then the implementation is:

void Foo::Fill()
{
    vec.push_back( L"abc aaa" );
    vec.push_back( L"def aaa" );
    vec.push_back( L"fed bbb" );
    vec.push_back( L"cba bbb" );
}

What I'd like to do to delete an element from this vector, lets say one that contains "def". What would be the easiest way to do so?

I'm thinking to use remove_if, but the comparator accept only 1 parameter - container element.

Is there an elegant solution? I'd use looping as a last resort.

remove_if expects expects a unary predicate. You can pass a lambda that returns true or false based upon your search. — Arunmu
– Arunmu, Commented Dec 16, 2016 at 19:39
Why do you need more than one argument for the predicate (I assume that's what you mean by comparator)? — MikeMB
– MikeMB, Commented Dec 16, 2016 at 19:40
@MikeMB, Sorry for the terminology mix-up. I expect to send the element and the string to compare the element to. Something like: bool pred( std::wstring elem, std::wstring element_to_remove); — Igor
– Igor, Commented Dec 16, 2016 at 19:43
@Igor: No problem. I believe LRiO already answered the question to your satisfaction? — MikeMB
– MikeMB, Commented Dec 16, 2016 at 19:50

Lightness Races in Orbit · Accepted Answer · 2016-12-16 19:47:43Z

4

If it's a class rather than a function, the predicate has a constructor, and constructors can take arguments.

You can pass your search string into this constructor, store it as a member, then use it inside the function call operator:

struct Pred
{
    Pred(std::wstring str) : str(std::move(str)) {}
    bool operator()(const std::wstring& el) const
    {
       return el.find(str) != el.npos;
    }
private:
    const std::wstring str;
};

std::remove_if(std::begin(vec), std::end(vec), Pred("def"));

// N.B. `el.find` is probably wrong. Whatever.

A "shortcut" to doing this is to use a lambda for the predicate, then the passing through of the search string is done for you either via the lambda capture, or by virtue of the fact that you literally typed it in right there and then:

std::remove_if(
   std::begin(vec), std::end(vec),
   [](const auto& el) { return el.find("def") != el.npos; }
);

// N.B. `el.find` is probably still wrong. Whatever.

If the predicate is a function, and you want it to invoke a binary comparator, you could mess about with std::bind to achieve the same result.

edited Dec 16, 2016 at 19:47

answered Dec 16, 2016 at 19:39

Lightness Races in Orbit

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5 Comments

Igor Over a year ago

I got a compiler error: "a parameter cannot have a type that contains 'auto'". After changing the "auto" to "std::wstring" everything compiles and runs fine. BTW, e1.find() is a correct function.

Lightness Races in Orbit Over a year ago

@Igor: You're using a compiler from the dark ages, apparently :) Yes, before C++14, you need to specify the type.

Igor Over a year ago

not really. I have MSVC 2010, which is enough for my purposes (C++11)

ildjarn Over a year ago

@Igor : A very limited subset of C++11... ;-]

Igor Over a year ago

@ildjarn, it is enough for my purpose so far... ;-)

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