CODEWITHSUNDEEP
c++arrayspointerschar
onestarblack
onestarblack

Reputation: 814

C++ function with char** parameter

I have some problems at understanding char pointers. Let's say there is the following method declaration (sorry but I don't have the implementation of the method, I hope it's enough to explain my problem):

short GetInfo(char **param1, char **param2);

I tried to call it 2 different ways:

// 1. When I call it this way it works
char param1_val[20];
char *param1_ptr = param1_val;

char param2_val[20];
char *param2_ptr = param2_val;

GetInfo(&param1_ptr, &param2_ptr);

// 2. When I call it that way I get an 'access violation exception'
char *param1_array[20];
char *param2_array[20];

GetInfo(param1_array, param2_array);

I thought 

char param1_val[20];
char *param1_ptr = param1_val;

is the same as

char *param1_array[20];

But it seems they are different. Is there a way how I could get my second case work? Or did I mix things up and I have to do it like in case 1?

Upvotes: 0

Views: 5955

Answers (2)

V-R
V-R

Reputation: 1401

The probable reason for declaring GetInfo as a function taking a char** is that it outputs a char*, so it is probably expected to be used more like so:

//short GetInfo(char **param1, char **param2);
char *ptr1=nullptr;
char *ptr2=nullptr;
GetInfo(&ptr1,&ptr2);
if(ptr1)    
    std::cout<<*ptr1<<std::endl; 
//etc

To really understand pointers and arrays, find C documentation on the subject. In C++, they are backwards compatible with C. However, in C++ it is better to work at higher level, e.g. use STL instead of coding in C style.

Upvotes: 2

Hatted Rooster
Hatted Rooster

Reputation: 36483

I thought

char param1_val[20];

char *param1_ptr = param1_val;

is the same as

char *param1_array[20];

They're not the same.

char param1_val[20];
char *param1_ptr = param1_val;

Declares an array of 20 chars param1_val and declares & assigns param1_ptr to the memory location of param1_val[0], so &param1_val[0].

char *param1_array[20];

Declares an array of 20 char pointers param1_array that still all point to garbage locations.

I'm assuming that GetInfo is dereferencing the pointers in your param1_array and that causes the access violation as trying to read from a random memory address is not something you want to do.

Upvotes: 4

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