修改传递给函数的指针时，Visual Studio C ++访问冲突

Question

I want to implement a simple function that gets a string as a char pointer and modifies the string in a function. The requested function must be void then I have to modify the primary string passed into my function. I got an access violation error and googled it but nothing helped.

My sample code is here:

#include "iostream"
using namespace std;
void FindCommonStr(char*& Common,int &A)
{   
    int i=0;
    while(1)
    {
        if(Common[i]=='\0')
            break;
        i++;
    }
    cout<<"Number of Elements = "<<i<<endl;
    for (int j=0 ; j<i-1;j++)   
        Common[j]='y';      
    A=2;    
}
void main()
{
    int A=0;    
    char* Common = new char;
    Common = "Hello World!";
    cout<<"Common0 = "<< Common<<endl;
    cout<<"A0 = "<< A<<endl;
    FindCommonStr(Common,A);    
    cout<<"Common1 = "<< Common<<endl;
    cout<<"A1 = "<< A<<endl;
}

Actually the problem occured in this part of FindCommonStr funtion:

for (int j=0 ; j<i-1;j++)   
            Common[j]='y';

If I comment this part everything works but I cannot change the string values. I also tested the pointer to pointer solution by defining the function as:

FindCommonStr(char **Common,...

It doesn't help though and I got the violation error again. Is it even possible to do such a thing?

Answer 1

When you do this:

Common = "Hello World!";

you are making the pointer Common point at a literal C-style string (and incidentally leaking the original char that you allocated via new previously). It is not valid to try to modify such a literal, so when you pass this to FindCommonStr and try to modify it you get an access violation.

You should avoid using C-style strings and use proper C++ std::string instead - this will save a lot of problems and is much more robust, as well as being more appropriate for C++ programming.

Fixed version of your code:

#include <iostream>
#include <string>

using namespace std;

static void FindCommonStr(string &Common, int &A)
{
    int i = 0;
    while (1)
    {
        if (Common[i] == '\0')
            break;
        i++;
    }
    cout << "Number of Elements = " << i << endl;
    for (int j = 0; j < i - 1; j++)
        Common[j] = 'y';
    A = 2;
}

int main()
{
    int A = 0;
    string Common = "Hello World!";
    cout << "Common0 = " << Common << endl;
    cout << "A0 = " << A << endl;
    FindCommonStr(Common, A);
    cout << "Common1 = " << Common<<endl;
    cout << "A1 = " << A << endl;
    return 0;
}

Alternatively if this is a homework assignment where you are required to use C strings for some unfathomable reason then a fixed version using only char * strings might look like this:

#include <iostream>

using namespace std;

static void FindCommonStr(char *Common, int &A)
{
    int i = 0;
    while (1)
    {
        if (Common[i] == '\0')
            break;
        i++;
    }
    cout << "Number of Elements = " << i << endl;
    for (int j = 0; j < i - 1; j++)
        Common[j] = 'y';
    A = 2;
}

int main()
{
    int A = 0;
    char Common[] = "Hello World!";
    cout << "Common0 = " << Common << endl;
    cout << "A0 = " << A << endl;
    FindCommonStr(Common, A);
    cout << "Common1 = " << Common<<endl;
    cout << "A1 = " << A << endl;
    return 0;
}

Answer 2

This part is conceptually wrong:

char* Common = new char;
// 'Common' is set to point to a piece of allocated memory
// (typically located in the heap)

Common = "Hello World!";
// 'Common' is set to point to a constant string
// (typically located in the code-section or in the data-section)

You are assigning variable Common twice, so obviously, the first assignment has no meaning.

It's like writing:

int i = 5;
i = 6;

On top of that, you "lose" the address of the allocated memory block, so you will not be able to release it at a later point in the execution of your program.

Answer 3

You seem to mixup char[] and string

When you write

char* Common = new char; 

you allocate space on the heap for one character which Common is pointing to.

Then you write

Common = "Hello World!";

which sets the pointer Common to point to the string "Hello World" in read-only memory. The heap you allocated previously is now leaked.

There are basically two approaches:

Either you work with arrays of characters, in that case you write something like

char* Common = new char[strlen("Hello World!")+1];
strcpy(Common, "Hello World!");

Now common still points to the heap and the string has been copied there. The extra +1 byte is to hold the ending \\0 string terminator.

You need to free the memory Common points to once you are done.

delete Common;

The other approach is to use the string template

std::string Common;

this allows you to assign a string, it capsules all the work with the heap array above.

Common = "Hello World!";

and there is no need to delete anything afterwards since std::string will do that for you.