C++优化之使用emplace

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我是靠谱客的博主冷静石头，这篇文章主要介绍C++优化之使用emplace，现在分享给大家，希望可以做个参考。

在C++开发过程中，我们经常会用STL的各种容器，比如vector，map，set等，这些容器极大的方便了我们的开发。在使用这些容器的过程中，我们会大量用到的操作就是插入操作，比如vector的push_back，map的insert，set的insert。这些插入操作会涉及到两次构造，首先是对象的初始化构造，接着在插入的时候会复制一次，会触发拷贝构造。但是很多时候我们并不需要两次构造带来效率的浪费，如果可以在插入的时候直接构造，就只需要构造一次就够了。

C++11标准已经有这样的语法可以直接使用了，那就是emplace。vector有两个函数可以使用：emplace，emplace_back。emplace类似insert，emplace_back类似push_back。通过示例代码可以更清晰的了解到他们的区别。

复制代码

// Book结构，保存书本信息
struct SBook
{
SBook() : bookName(""), price(0)
{
std::cout << "default construct: " << bookName << std::endl;
}
SBook(std::string bookName_, int price_) : bookName(bookName_), price(price_)
{
std::cout << "construct: " << bookName << std::endl;
};
SBook(SBook& rhs) : bookName(rhs.bookName), price(rhs.price)
{
std::cout << "copy construct: " << bookName << std::endl;
}
~SBook()
{
std::cout << "deconstruct: " << bookName << std::endl;
}
bool operator <(const SBook& rhs) const
{
return bookName < rhs.bookName;
}
std::string bookName;
int price;
};
// 测试vector
vector<SBook> books;
// 预先分配，否则整个vector在容量不够的情况下重新分配内存
books.reserve(100);
std::cout << "test push_back:" << endl;
books.push_back(SBook("C++从入门到放弃", 1));
std::cout << endl;
std::cout << "test emplace_back:" << endl;
books.emplace_back("水浒传", 2);
std::cout << endl;
std::cout << "test emplace_back default:" << endl;
books.emplace_back();
auto& book = books.back();
book.bookName = "红楼梦";
book.price = 5;
std::cout << endl;
std::cout << "test emplace:" << endl;
auto it = books.emplace(books.end());
it->bookName = "西游记";
it->price = 3;
std::cout << endl;
std::cout << "output all books: " << endl;
for_each(books.begin(), books.end(), [](const SBook& book)->void
{
std::cout << book.bookName << endl;
});
std::cout << endl;

// 测试set
set<SBook> bookSet;

std::cout << "test bookSet insert:" << endl;
bookSet.insert(SBook("十万个为什么", 1));
std::cout << endl;

std::cout<< "test bookSet emplace:" << endl;
bookSet.emplace("新华字典", 2);
std::cout << endl;

std::cout<< "output bookset: " << endl;
for_each(bookSet.begin(), bookSet.end(), [](const SBook&book)->void
{
std::cout << book.bookName << endl;
});
std::cout << endl;

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// Book结构，保存书本信息
struct SBook
{
SBook() : bookName(""), price(0)
{
std::cout << "default construct: " << bookName << std::endl;
}
SBook(std::string bookName_, int price_) : bookName(bookName_), price(price_)
{
std::cout << "construct: " << bookName << std::endl;
};
SBook(SBook& rhs) : bookName(rhs.bookName), price(rhs.price)
{
std::cout << "copy construct: " << bookName << std::endl;
}
~SBook()
{
std::cout << "deconstruct: " << bookName << std::endl;
}
bool operator <(const SBook& rhs) const
{
return bookName < rhs.bookName;
}
std::string bookName;
int price;
};
// 测试vector
vector<SBook> books;
// 预先分配，否则整个vector在容量不够的情况下重新分配内存
books.reserve(100);
std::cout << "test push_back:" << endl;
books.push_back(SBook("C++从入门到放弃", 1));
std::cout << endl;
std::cout << "test emplace_back:" << endl;
books.emplace_back("水浒传", 2);
std::cout << endl;
std::cout << "test emplace_back default:" << endl;
books.emplace_back();
auto& book = books.back();
book.bookName = "红楼梦";
book.price = 5;
std::cout << endl;
std::cout << "test emplace:" << endl;
auto it = books.emplace(books.end());
it->bookName = "西游记";
it->price = 3;
std::cout << endl;
std::cout << "output all books: " << endl;
for_each(books.begin(), books.end(), [](const SBook& book)->void
{
std::cout << book.bookName << endl;
});
std::cout << endl;


// 测试set
set<SBook> bookSet;

std::cout << "test bookSet insert:" << endl;
bookSet.insert(SBook("十万个为什么", 1));
std::cout << endl;

std::cout<< "test bookSet emplace:" << endl;
bookSet.emplace("新华字典", 2);
std::cout << endl;

std::cout<< "output bookset: " << endl;
for_each(bookSet.begin(), bookSet.end(), [](const SBook&book)->void
{
std::cout << book.bookName << endl;
});
std::cout << endl;