我能够为std::list和std::vector编写一个通用的包含函数，

template<typename T>
bool contains( const list<T>& container, const T& elt )
{
  return find( container.begin(), container.end(), elt ) != container.end() ;
}

template<typename T>
bool contains( const vector<T>& container, const T& elt )
{
  return find( container.begin(), container.end(), elt ) != container.end() ;
}

// use:
if( contains( yourList, itemInList ) ) // then do something

这样可以稍微清理一下语法。

但是我不能使用模板模板参数魔术使此工作任意stl容器。

// NOT WORKING:
template<template<class> class STLContainer, class T>
bool contains( STLContainer<T> container, T elt )
{
  return find( container.begin(), container.end(), elt ) != container.end() ;
}

任何关于改进上一个答案的评论都是很好的。

就是这个，一英里。

bool once(uintptr_t val) {
    return visited.emplace(val).second;
}

怎么会不是这样呢?

https://godbolt.org/z/9zP77jqMc

func5(unsigned long):
        sub     rsp, 24
        mov     QWORD PTR [rsp+8], rdi
        lea     rsi, [rsp+8]
        mov     edi, OFFSET FLAT:visited2
        call    std::pair<std::_Rb_tree_iterator<unsigned long>, bool> std::_Rb_tree<unsigned long, unsigned long, std::_Identity<unsigned long>, std::less<unsigned long>, std::allocator<unsigned long> >::_M_emplace_unique<unsigned long&>(unsigned long&)
        add     rsp, 24
        mov     eax, edx
        ret

如果你要添加一个contains函数，它可能看起来像这样:

#include <algorithm>
#include <iterator>

template<class TInputIterator, class T> inline
bool contains(TInputIterator first, TInputIterator last, const T& value)
{
    return std::find(first, last, value) != last;
}

template<class TContainer, class T> inline
bool contains(const TContainer& container, const T& value)
{
    // This works with more containers but requires std::begin and std::end
    // from C++0x, which you can get either:
    //  1. By using a C++0x compiler or
    //  2. Including the utility functions below.
    return contains(std::begin(container), std::end(container), value);

    // This works pre-C++0x (and without the utility functions below, but doesn't
    // work for fixed-length arrays.
    //return contains(container.begin(), container.end(), value);
}

template<class T> inline
bool contains(const std::set<T>& container, const T& value)
{
    return container.find(value) != container.end();
}

这适用于std::set，其他STL容器，甚至固定长度的数组:

void test()
{
    std::set<int> set;
    set.insert(1);
    set.insert(4);
    assert(!contains(set, 3));

    int set2[] = { 1, 2, 3 };
    assert(contains(set2, 3));
}

编辑:

正如评论中指出的，我无意中使用了c++ 0x的新函数(std::begin和std::end)。下面是VS2010中近乎琐碎的实现:

namespace std {

template<class _Container> inline
    typename _Container::iterator begin(_Container& _Cont)
    { // get beginning of sequence
    return (_Cont.begin());
    }

template<class _Container> inline
    typename _Container::const_iterator begin(const _Container& _Cont)
    { // get beginning of sequence
    return (_Cont.begin());
    }

template<class _Container> inline
    typename _Container::iterator end(_Container& _Cont)
    { // get end of sequence
    return (_Cont.end());
    }

template<class _Container> inline
    typename _Container::const_iterator end(const _Container& _Cont)
    { // get end of sequence
    return (_Cont.end());
    }

template<class _Ty,
    size_t _Size> inline
    _Ty *begin(_Ty (&_Array)[_Size])
    { // get beginning of array
    return (&_Array[0]);
    }

template<class _Ty,
    size_t _Size> inline
    _Ty *end(_Ty (&_Array)[_Size])
    { // get end of array
    return (&_Array[0] + _Size);
    }

}

Just to clarify, the reason why there is no member like contains() in these container types is because it would open you up to writing inefficient code. Such a method would probably just do a this->find(key) != this->end() internally, but consider what you do when the key is indeed present; in most cases you'll then want to get the element and do something with it. This means you'd have to do a second find(), which is inefficient. It's better to use find directly, so you can cache your result, like so:

auto it = myContainer.find(key);
if (it != myContainer.end())
{
    // Do something with it, no more lookup needed.
}
else
{
    // Key was not present.
}

当然，如果你不关心效率，你总是可以自己滚动，但在这种情况下，你可能不应该使用c++…；）

在c++ 20中，我们最终得到std::set::contains方法。

#include <iostream>
#include <string>
#include <set>

int main()
{
    std::set<std::string> example = {"Do", "not", "panic", "!!!"};

    if(example.contains("panic")) {
        std::cout << "Found\n";
    } else {
        std::cout << "Not found\n";
    }
}

您还可以在插入元素时检查元素是否在set中。 单元素版本返回一个pair，其成员pair::first set指向一个迭代器，该迭代器要么指向新插入的元素，要么指向集合中已经存在的等效元素。如果插入了新元素，pair中的第二个元素将被设置为true，如果已经存在等效元素则为false。

例如:假设集合中已经有20作为元素。

 std::set<int> myset;
 std::set<int>::iterator it;
 std::pair<std::set<int>::iterator,bool> ret;

 ret=myset.insert(20);
 if(ret.second==false)
 {
     //do nothing

 }
 else
 {
    //do something
 }

 it=ret.first //points to element 20 already in set.

如果元素是新插入的，则than pair::first将指向新元素在set中的位置。