[英]How to properly cast char ** x to const char **
I have the following variable.我有以下变量。
char **arr
Then I want to perform some modification on array which means it can't be declared as a constant.然后我想对数组进行一些修改,这意味着它不能被声明为常量。
Now I have a function that accepts the argument of type const char ** arr
.现在我有一个 function 接受
const char ** arr
类型的参数。 But I have no control over the signature of this function.但是我无法控制这个 function 的签名。
Now when I cast arr
to const char ** arr
g++ generates a warning which is [-Werror=cast-qual]
.现在,当我将
arr
转换为const char ** arr
g++ 时会生成一个警告,即[-Werror=cast-qual]
。
For more clarification consider following MCVE:有关更多说明,请考虑关注 MCVE:
#include<cstdio>
void print(const char** x){
printf("%s", x[0]);
}
int main(int argc, char **argv){
if(argc>1){
print((const char **)argv);
}
return 0;
}
//Then compile it as follow:
$ g++ -Wcast-qual test.cpp
//gives the following output:
MCVE.cpp: In function ‘int main(int, char**)’:
MCVE.cpp:5:36: warning: cast from type ‘char**’ to type ‘const char**’ casts away qualifiers [-Wcast-qual]
const char ** q = (const char**) argv;
So my question is why this generates a warning?所以我的问题是为什么这会产生警告? Is there any risk in doing this?
这样做有什么风险吗?
And how to achieve a behavior I want to achieve?以及如何实现我想要实现的行为?
Allowing a cast from char**
to const char**
provides a loophole to modify a const char
or a const char*
.允许从
char**
转换为const char**
提供了修改const char
或const char*
的漏洞。
Sample code:示例代码:
const char c = 'A';
void foo(const char** ptr)
{
*ptr = &c; // Perfectly legal.
}
int main()
{
char* ptr = nullptr;
foo(&ptr); // When the function returns, ptr points to c, which is a const object.
*ptr = 'B'; // We have now modified c, which was meant to be a const object.
}
Hence, casting a char**
to const char**
is not a safe cast.因此,将
char**
转换为const char**
不是安全的转换。
You can use您可以使用
if(argc>1)
{
const char* ptr = argv[0];
print(&ptr);
}
for your code to compile without the cast-qual
warning.让您的代码在没有
cast-qual
警告的情况下编译。
If you need to pass more than just the first argument, you'll need to construct an array of const char*
and use it.如果您需要传递的不仅仅是第一个参数,则需要构造一个
const char*
数组并使用它。
if(argc>1)
{
int N = <COMPUTE N FIRST>;
const char** ptr = new const char*[N];
for (int i = 0; i < N; ++i )
{
ptr[i] = argv[i];
}
print(ptr);
delete [] ptr; // Make sure to deallocate dynamically allocated memory.
}
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