第一次嘗試寫單例。這是一個計時器，它與一個功能一起處理計時器的啟動和停止以及列印結果。編譯時，我收到這樣的聯結器錯誤：
:-1: ошибка: CMakeFiles/some_algorithms.dir/timer_singleton.cpp.obj:timer_singleton.cpp:(.rdata$.refptr._ZN15timer_singleton7counterE[.refptr._ZN15timer_singleton7counterE] 0x0): undefined reference to `timer_singleton::counter'
是什么導致了這個錯誤，我該如何解決？
這是我的源代碼：
timer_singleton.h
#ifndef TIMER_SINGLETON_H
#define TIMER_SINGLETON_H
#pragma once
#include <iostream>
#include <chrono>
class timer_singleton
{
public:
timer_singleton(timer_singleton & other) = delete;
void operator=(const timer_singleton& other) = delete;
static timer_singleton * getInstance();
static void hit_the_clock();
private:
timer_singleton();
static timer_singleton * instance;
static std::chrono::high_resolution_clock clock;
static std::chrono::high_resolution_clock::time_point start;
static std::chrono::high_resolution_clock::time_point stop;
static size_t counter;
};
#endif // TIMER_SINGLETON_H
timer_singleton.cpp
#include "timer_singleton.h"
timer_singleton::timer_singleton()
{
clock = std::chrono::high_resolution_clock();
start = clock.now();
stop = clock.now();
counter = 0;
}
timer_singleton * timer_singleton::getInstance()
{
if (instance == nullptr)
{
instance = new timer_singleton();
}
return instance;
}
void timer_singleton::hit_the_clock()
{
if (counter % 2 == 1)
{
// Clocks start ticking
start = clock.now();
counter;
}
else
{
// Clocks stop ticking and print time measured time
stop = clock.now();
auto duration = std::chrono::duration_cast<std::chrono::microseconds>(stop - start);
std::cout << "Measured time = " << duration.count() << " microseconds" << std::endl;
counter;
}
}
主檔案
#include "timer_singleton.h"
// ...
timer_singleton * timer = timer_singleton::getInstance();
timer->hit_the_clock();
// some calculations
timer->hit_the_clock();
uj5u.com熱心網友回復：
問題：大多數非常量static成員需要在類定義之外定義，以便獲得將由所有類實體共享的唯一一個實體。通常這意味著在 timer_singleton.cpp 你必須添加
timer_singleton::counter = 0; // allocate and initialize
但...
單例已經有效static，所以唯一的靜態成員應該是獲取實體的函式。這使整個問題消失了。
帶有關于其他有用更改的注釋的新代碼：
class timer_singleton
{
public:
timer_singleton(timer_singleton &other) = delete;
void operator=(const timer_singleton &other) = delete;
static timer_singleton* getInstance();
void hit_the_clock(); // shouldn't be static
private:
timer_singleton();
// None of these should have been static
std::chrono::high_resolution_clock clock; // This clock could jump around,
// including backward, in time.
// Safer with a steady_clock
std::chrono::high_resolution_clock::time_point start;
std::chrono::high_resolution_clock::time_point stop;
size_t counter;
};
timer_singleton::timer_singleton():
start(clock.now()),
stop(start), // guaranteed to be same as start
counter(0)
{ // assignments replaced with initializations in member initializer list
}
timer_singleton* timer_singleton::getInstance()
{ // now using Meyers singelton
static timer_singleton instance;
return &instance; // consider adjusting to return a reference.
// Often a bit cleaner thanks to the no null guarantee
}
void timer_singleton::hit_the_clock()
{
auto now = clock.now(); // if timing is critical, the first thing
// you do is get the current time.
//if (counter % 2 == 1) // remember counter starts at 0, so first hit
// would stop, not start, the timer.
if (counter % 2 == 0)
{
// Clocks start ticking
start = now;
}
else
{
// Clocks stop ticking and print time measured time
stop = now;
auto duration = std::chrono::duration_cast<std::chrono::microseconds>(stop - start);
std::cout << "Measured time = " << duration.count() << " microseconds" << std::endl;
}
counter; // don't repeat yourself
}
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