std::list里面的push_back和resize效率问题

张某人的胡思乱想2024-08-06 13:15

2024年8月6日：

向list里面循环添加数据，两种写法

cpp 复制代码

for(;;){
  myList.resize(myList.size()+1);
  MyObject &obj= *myList.rbegin();
  obj.a= itervalue;
}

第二种写法是push_back:

cpp 复制代码

for(;;){
  MyObject obj；
  obj.a= itervalue;
  myList.push_back(obj);
}

用第一种用法，是避免了push_back的拷贝构造函数，用的缺省构造函数，对于COPY构造函数效率毕竟低的场景，也许有效。但是大部分时间应该没有啥用。

效率上，找了一个评测网站：

Quick C++ Benchmarks

代码：

cpp 复制代码

#include <vector>
#include <list>

constexpr size_t loopCount = 1000;

struct BigObject{
    double LongDouble[10000];
};

static void VectorPushBack(benchmark::State& state) {
  for (auto _ : state) {
    std::vector<BigObject> vec;
    for (int i = 0; i < loopCount; i++) {
      BigObject bo;
      vec.push_back(bo);
    }
    benchmark::DoNotOptimize(vec);
  }
}
BENCHMARK(VectorPushBack);

static void VectorEmplaceBack(benchmark::State& state) {
  for (auto _ : state) {
    std::vector<BigObject> vec;
    for (int i = 0; i < loopCount; i++) {
      vec.emplace_back();
    }
    benchmark::DoNotOptimize(vec);
  }
}
BENCHMARK(VectorEmplaceBack);

static void ListPushBack(benchmark::State& state) {
  for (auto _ : state) {
    std::list<BigObject> list;
    for (int i = 0; i < loopCount; i++) {
      BigObject bo;
      list.push_back(bo);
    }
    benchmark::DoNotOptimize(list);
  }
}
BENCHMARK(ListPushBack);

static void ListEmplaceBack(benchmark::State& state) {
  for (auto _ : state) {
    std::list<BigObject> list;
    for (int i = 0; i < loopCount; i++) {
      list.emplace_back();
    }
    benchmark::DoNotOptimize(list);
  }
}
BENCHMARK(ListEmplaceBack);
static void ListResize(benchmark::State& state) {
  for (auto _ : state) {
    std::list<BigObject> list;
    for (int i = 0; i < loopCount; i++) {
      BigObject bo;
      //list.push_back(bo);
      list.resize(list.size()+1);
      BigObject &obj = *list.rbegin();
      obj=bo;
    }
    benchmark::DoNotOptimize(list);
  }
}
BENCHMARK(ListResize);

结果：push_back效率最高，高的原因不详

参考：

https://www.reddit.com/r/cpp_questions/comments/15uekey/push_back_big_object_into_stdvector_by_creating/?rdt=60610