C++ 性能优化 - for循环条件中不要调用函数

1. for循环条件中调用普通函数

#include <iostream>
#include <chrono>
using namespace std;

long long count = 0;
const int N = 10;
int getSize(){
    cout<<"get size"<<endl;
    return N + 1;
}

void timeMeasure(void(*f)()){
    auto begin = std::chrono::high_resolution_clock::now();
    uint32_t iterations = 1;
    for(uint32_t i = 0; i < iterations; ++i)
    {
        f();
    }
    auto end = std::chrono::high_resolution_clock::now();
    auto duration = std::chrono::duration_cast<std::chrono::nanoseconds>(end-begin).count();
    std::cout << duration << "ns total, average : " << duration / iterations << "ns." << std::endl;
}


void test1(){
    for(int i = 0; i < getSize(); ++i){
        count++;
    }
}

void test2(){
    for(int i = 0; i < N + 1; ++i){
        count++;
    }
}

int main(){
    timeMeasure(test1);
    cout<<count<<endl;
    timeMeasure(test2);
    cout<<count<<endl;
    return 0;
}

8642ns total, average : 86ns.
1000
6502ns total, average : 65ns.
2000

2. 探究vector size()函数

#include <iostream>
#include <chrono>
#include <vector>
using namespace std;
long long count = 0;
const int N = 100;
vector<int> arr1;
vector<int> arr2;
void timeMeasure(void(*f)()){
    auto begin = std::chrono::high_resolution_clock::now();
    uint32_t iterations = 2;
    for(uint32_t i = 0; i < iterations; ++i)
    {
        f();
    }
    auto end = std::chrono::high_resolution_clock::now();
    auto duration = std::chrono::duration_cast<std::chrono::nanoseconds>(end-begin).count();
    std::cout << duration << "ns total, average : " << duration / iterations << "ns." << std::endl;
}


void test1(){
    for(int i = 0; i < arr1.size(); ++i){
        count++;
    }
}

void test2(){
    int num = arr2.size();
    for(int i = 0; i < num; ++i){
        count++;
    }
}

int main(){
    arr1.assign(N, 0);
    arr2.assign(N, 0);
    timeMeasure(test1);
    cout<<count<<endl;
    timeMeasure(test2);
    cout<<count<<endl;
    return 0;
}

3412ns total, average : 1706ns.
200
1675ns total, average : 837ns.
400