c++ - 如何使我的动态数组或 vector 以与标准数组相似的速度运行？ C++

Question

我在 C++ 方面仍然缺乏经验，我正在尝试编写求和代码以精确地添加数字。这是一些有限差分软件的 dll 插件，代码在运行期间被调用了数百万次。我想编写一个函数，可以传入任意数量的参数并返回总和。我的代码如下所示:

#include <cstdarg>

double SumFunction(int numArgs, ...){ // this allows me to pass any number 
                                      // of arguments to my function.
va_list args;
va_start(args,numArgs); //necessary prerequisites for using cstdarg

double myarray[10];
for (int i = 0; i < numArgs; i++) {
    myarray[i] = va_arg(args,double);
}       // I imagine this is sloppy code; however i cannot create
        // myarray{numArgs] because numArgs is not a const int.
sum(myarray); // The actual method of addition is not relevant here, but
              //for more complicated methods, I need to put the summation 
              // terms in a list.

vector<double> vec(numArgs); // instead, place all values in a vector
for (int i = 0; i < numArgs; i++) {
    vec.at(i) = va_arg(args,double);
}
sum(vec); //This would be passed by reference, of course. The function sum
          // doesn't actually exist, it would all be contained within the 
          // current function. This is method is twice as slow as placing 
          //all the values in the static array.

double *vec;
vec =  new double[numArgs];
for (int i = 0; i < (numArgs); i++) {
    vec[i] = va_arg(args,double);
}
sum(vec); // Again half of the speed of using a standard array and 
          // increasing in magnitude for every extra dynamic array!

delete[] vec;
va_end(args);
}

所以我遇到的问题是，使用过大的静态数组是草率的编程，但使用 vector 或动态数组会大大降低程序速度。所以我真的不知道该怎么办。请问有人可以帮忙吗？

Answer 1

加快代码速度(代价是使其变得更复杂)的一种方法是在调用之间重用动态数组或 vector ，这样就可以避免每次调用函数时产生内存分配和释放的开销。

例如，在您的函数外将这些变量声明为全局变量或某个类中的成员变量。为了便于解释，我将它们设为全局变量:

double* sumArray = NULL;
int sumArraySize = 0;

在您的 SumFunction 中，检查数组是否存在，如果不存在则分配它，并在必要时调整大小:

double SumFunction(int numArgs, ...){ // this allows me to pass any number 
                                  // of arguments to my function.
    va_list args;
    va_start(args,numArgs); //necessary prerequisites for using cstdarg

    // if the array has already been allocated, check if it is large enough and delete if not:
    if((sumArray != NULL) && (numArgs > sumArraySize))
    {
        delete[] sumArray;
        sumArray = NULL;
    }

    // allocate the array, but only if necessary:
    if(sumArray == NULL)
    {
        sumArray = new double[numArgs];
        sumArraySize = numArgs;
    }

    double *vec = sumArray;   // set to your array, reusable between calls
    for (int i = 0; i < (numArgs); i++) {
        vec[i] = va_arg(args,double);
    }
    sum(vec, numArgs); // you will need to pass the array size

    va_end(args);

    // note no array deallocation
}

要注意的是，您需要记住在某个时候通过调用与此类似的函数来释放数组(就像我说的那样，您需要为速度付出额外的复杂性):

void freeSumArray()
{
    if(sumArray != NULL)
    {
        delete[] sumArray;
        sumArray = NULL;
        sumArraySize = 0;
    }
}

您可以对 vector 采用类似(且更简单/更清晰)的方法，如果它不存在则在第一次分配它，或者如果它存在则使用 numArgs 对其调用 resize()。