C++ std::set emplace 返回值 first second

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软件工程小施同学 2022-01-30 16:58:44 博主文章分类：C++ ©著作权

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emplace返回值

一个pair

逻辑组件如果已插入那就是真的, 如果映射已经包含值相同地排序的元素就是假的.
返回值的迭代器元素对返回插入新元素的地址 (如果 bool 元素为 true) 或已找到其中的元素 (如果 bool 元素是假)。

If the function successfully inserts the element (because no equivalent element existed already in the set), the function returns a pair of an iterator to the newly inserted element and a value of true.

Otherwise, it returns an iterator to the equivalent element within the container and a value of false.

Member type iterator is a bidirectional iterator type that points to an element.

pair is a class template declared in <utility> (see pair).

set::emplace - C++ Reference

// set_emplace.cpp
// compile with: /EHsc
#include <set>
#include <string>
#include <iostream>

using namespace std;

template <typename S> void print(const S& s) {
    cout << s.size() << " elements: ";

    for (const auto& p : s) {
        cout << "(" << p << ") ";
    }

    cout << endl;
}

int main()
{
    set<string> s1;

    auto ret = s1.emplace("ten");

    if (!ret.second){
        cout << "Emplace failed, element with value \"ten\" already exists."
            << endl << "  The existing element is (" << *ret.first << ")"
            << endl;
        cout << "set not modified" << endl;
    }
    else{
        cout << "set modified, now contains ";
        print(s1);
    }
    cout << endl;

    ret = s1.emplace("ten");

    if (!ret.second){
        cout << "Emplace failed, element with value \"ten\" already exists."
            << endl << "  The existing element is (" << *ret.first << ")"
            << endl;
    }
    else{
        cout << "set modified, now contains ";
        print(s1);
    }
    cout << endl;
}