[英]C++ passing reference to array to C function
I'm trying to call a function written in C which passes in a pointer to an array. 我试图调用用C编写的函数,该函数将指针传递给数组。
In C++ I have the following: 在C ++中,我具有以下内容:
double* x = new double[10];
populateArray(x);
In C: 在C中:
void populateArray(double* vars);
{
vars = (double*) malloc(10*sizeof(double));
int i;
for(i = 0; (i < 10); i++)
{
vars[i] = 1*20;
}
}
However, when I try to output the contents of x
inside the C++ code, the result is always 0? 但是,当我尝试在C ++代码中输出
x
的内容时,结果始终为0?
Problem arises because you are changing local variable vars
. 出现问题是因为您正在更改局部变量
vars
。
Change to: 改成:
double* x; //you don't need to allocate here if you are allocating later
populateArray(&x);
And: 和:
void populateArray(double** vars)
{
*vars = malloc(10*sizeof(double)); //also no need to cast malloc
int i;
for(i = 0; (i < 10); i++)
{
(*vars)[i] = 1*20;
}
}
void populateArray(double* vars);
{
vars = (double*) malloc(10*sizeof(double)); // <---- remove this line completely
The problem is you're ignoring the vars
you were given and creating a brand new buffer for it. 问题是您忽略了给定的
vars
,并为其创建了一个全新的缓冲区。
Worse, that buffer pointer gets lost when this function returns, causing a memory leak. 更糟糕的是,此函数返回时,缓冲区指针丢失,从而导致内存泄漏。
Just use the vars
you were given -- delete the line indicated above. 只需使用您获得的
vars
请删除上面指示的行。
Short answer: Remove the malloc
call inside the function. 简短答案:删除函数内部的
malloc
调用。
Slightly longer answer: 答案略长:
You're changing the value of the vals
pointer to another newly allocated memory -- thus, the argument you pass in gets unused (and thus is never populated). 您正在将
vals
指针的值更改为另一个新分配的内存-因此,您传入的参数将变为未使用的(因此永远不会填充)。
The result being always 0 is a coincidence, it could be anything (or you could end up with nasal demons ) because you're then reading uninitialized memory, which is undefined behavior. 结果始终为0是一个巧合,它可以是任何东西(或者您可能会患上鼻恶魔 ),因为您随后将读取未初始化的内存,这是未定义的行为。
This of it as this, if you remove the call: 如果您删除呼叫,则如下所示:
vals = new double[10];
vals = malloc(10 * sizeof(double));
The first value is overwritten. 第一个值将被覆盖。
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