如何让C++字符串更快
1、上代码，原有写法
#include <iostream>
#include <string>
static uint32_t s_AllocCount = 0;
void* operator new(size_t size)
{
s_AllocCount++;
std::cout << "Allocating " << size << " bytes\n";
return malloc(size);
}
void PrintName(const std::string& name)
{
std::cout << name << std::endl;
}
int main()
{
std::string name = "Yan Chernikov";
PrintName(name);
std::string firstName = name.substr(0, 3);
std::string lastName = name.substr(4, 9);
std::cout << s_AllocCount << " allocations" << std::endl;
std::cin.get();
}1.1、运行结果
分配了三次内存
std::string name ="Yan Chernikov";
PrintName(name);
std::string firstName = name.substr(0,3);
std::string lastName = name.substr(4,9);
2、第一次优化
2.1、代码
#include <iostream>
#include <string>
static uint32_t s_AllocCount = 0;
void* operator new(size_t size)
{
s_AllocCount++;
std::cout << "Allocating " << size << " bytes\n";
return malloc(size);
}
void PrintName(std::string_view name)
{
std::cout << name << std::endl;
}
int main()
{
std::string name = "Yan Chernikov";
#if 0
std::string firstName = name.substr(0, 3);
std::string lastName = name.substr(4, 9);
#else
std::string_view firstName(name.c_str(), 3);
std::string_view lastName(name.c_str() + 4, 9);
#endif
PrintName(firstName);
PrintName(lastName);
std::cout << s_AllocCount << " allocations" << std::endl;
std::cin.get();
}2.2 运行结果
image.png
2.3、用string_view 替换string
3、第二次优化
3.1、代码
#include <iostream>
#include <string>
static uint32_t s_AllocCount = 0;
void* operator new(size_t size)
{
s_AllocCount++;
std::cout << "Allocating " << size << " bytes\n";
return malloc(size);
}
void PrintName(std::string_view name)
{
std::cout << name << std::endl;
}
int main()
{
const char* name = "Yan Chernikov";
#if 0
std::string firstName = name.substr(0, 3);
std::string lastName = name.substr(4, 9);
#else
std::string_view firstName(name, 3);
std::string_view lastName(name + 4, 9);
#endif
PrintName("firstName");
PrintName("lastName");
std::cout << s_AllocCount << " allocations" << std::endl;
std::cin.get();
}3.2、运行结果
image.png
3.3、修改的代码
const char* name = "Yan Chernikov";std::string_view firstName(name, 3);std::string_view lastName(name + 4, 9);PrintName("firstName");PrintName("lastName");4、第二次修改(const char* name = "Yan Chernikov";)在只能使用string且数据量较大的情况下，是不方便的。
我们要做的是修改PrintName函数中的参数为PrintName(std::string_view nam),分配次数就可以从1变成0了。
4.1、代码
#include <iostream>
#include <string>
static uint32_t s_AllocCount = 0;
void* operator new(size_t size)
{
s_AllocCount++;
std::cout << "Allocating " << size << " bytes\n";
return malloc(size);
}
void PrintName(std::string_view name)
{
std::cout << name << std::endl;
}
int main()
{
const char* name = "Yan Chernikov";
#if 0
std::string firstName = name.substr(0, 3);
std::string lastName = name.substr(4, 9);
#else
std::string_view firstName(name, 3);
std::string_view lastName(name + 4, 9);
#endif
PrintName(name);
/*PrintName("firstName");
PrintName("lastName");*/
std::cout << s_AllocCount << " allocations" << std::endl;
std::cin.get();
}4.2、运行结果：
image.png
5、总结
像这种在C++中，尤其是多个循环中，使用string是非常影响性能的，如果可以使用string_view，则可以有效提示效率，加快运行速度。
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本文系外文翻译，前往查看
如有侵权，请联系 cloudcommunity@tencent.com 删除。
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