分配内存后是否需要初始化变量？

Question

我正在尝试用 C++ 实现矩阵乘法。我找到了使用 class 写入 .h 和 .cpp 文件的示例代码。这只是与我的问题相关的部分代码：

#include "Matrix.h"

// Constructor - using an initialisation list here
Matrix::Matrix(int rows, int cols, bool preallocate): rows(rows), cols(cols), size_of_values(rows * cols), preallocated(preallocate)
{
   // If we want to handle memory ourselves
   if (this->preallocated)
   {
      // Must remember to delete this in the destructor
      this->values = new double[size_of_values];
   }
}

void Matrix::matMatMult(Matrix& mat_left, Matrix& output)
{
   // The output hasn't been preallocated, so we are going to do that

      output.values = new double[this->rows * mat_left.cols];

   // Set values to zero before hand
   for (int i = 0; i < output.size_of_values; i++)
   {
      output.values[i] = 0;
   }

我想知道为什么他们使用带有 0 的输出矩阵初始化 output.values[i] = 0; 而它之前已经分配了内存？

Answer 1

从 cppreference 到 new 表达式：

The object created by a new-expression is initialized according to the following rules:

[...]

If type is an array type, an array of objects is initialized.

If initializer is absent, each element is default-initialized

If initializer is an empty pair of parentheses, each element is value-initialized.

“默认初始化”int通俗地说是未初始化。它们具有不确定的值。空括号指的是 Ted 在评论中提到的内容：

output.values = new double[this->rows * mat_left.cols]{};

描述了值初始化here。这里适用的案例是

otherwise, the object is zero-initialized.

I wonder why they initialised using the output matrix with 0s output.values[i] = 0; while it has been allocated memory before?

分配内存和初始化对象是两个独立的步骤。是的，元素要初始化，分配内存不够

分配内存后是否需要初始化变量？

Does it need to initialise the variable after allocating memory?

c++

memory

matrix-multiplication