这两段代码有什么区别?
struct HighResClock
{
typedef long long rep;
typedef std::nano period;
typedef std::chrono::duration<rep, period> duration;
typedef std::chrono::time_point<HighResClock> time_point;
static const bool is_steady = true;
static time_point now();
};
namespace
{
auto g_Frequency = []() -> long long
{
std::cout<<"HERE";
LARGE_INTEGER frequency;
QueryPerformanceFrequency(&frequency);
return frequency.QuadPart;
}();
}
HighResClock::time_point HighResClock::now()
{
LARGE_INTEGER count;
QueryPerformanceCounter(&count);
return time_point(duration(count.QuadPart * static_cast<rep>(period::den) / g_Frequency));
}
int main()
{
HighResClock c;
c.now();
c.now();
c.now();
}
和
struct HighResClock
{
typedef long long rep;
typedef std::nano period;
typedef std::chrono::duration<rep, period> duration;
typedef std::chrono::time_point<HighResClock> time_point;
static const bool is_steady = true;
static time_point now();
};
namespace
{
auto g_Frequency = []() -> long long
{
std::cout<<"HERE";
LARGE_INTEGER frequency;
QueryPerformanceFrequency(&frequency);
return frequency.QuadPart;
};
}
HighResClock::time_point HighResClock::now()
{
LARGE_INTEGER count;
QueryPerformanceCounter(&count);
return time_point(duration(count.QuadPart * static_cast<rep>(period::den) / g_Frequency()));
}
int main()
{
HighResClock c;
c.now();
c.now();
c.now();
}
如果你没有注意到,不同之处在于下面的括号:
auto g_Frequency = []() -> long long
{
LARGE_INTEGER frequency;
QueryPerformanceFrequency(&frequency);
return frequency.QuadPart;
}(); //this bracket here appears in one and not the other..
我问是因为带括号的只打印一次“这里”,而另一个(没有括号)打印 3 次。括号是什么意思,它有什么作用?这种带括号的语法有名称吗?
最佳答案
在 lambda 定义 []{}();
之后立即写入 ()
将调用 lambda,因此结果是 lambda 返回类型的类型.
如果省略 ()
后缀,将返回一个 lambda 类型(未指定),它基本上是一个可调用仿函数。
auto result = []{ return 42; }(); // auto is integer, result has 42 in it
auto result1 = []{ return 42; }; // auto is some unspecified lambda type
auto result2 = result1(); // auto is integer, result2 is storing 42`
......................^^ - this is the bracket you can also put directly after the definition of the lambda
关于C++ lambda 后跟 () 与没有 () 的 lambda,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/20852806/