c++ Accessing members of objects using pointers

Question

I have variable mode, which I am declaring with the following line:

StatusRecord mode;

StatusRecord is a struct which holds several variables of different types.

I now want to create a pointer to mode, and populate that pointer with some data by using a function to populate its attributes/ fields. I have tried doing this as follows:

StatusRecord mode;
StatusRecord *modePtr = &mode;
DataStore->getModeData(*modePtr);

Here, I am declaring the struct variable, creating a pointer to it, and populating that pointer using the getModeData() function. However, I now want to use an attribute of the struct ptr I've just populated in a conditional statement:

if(*modePtr->eraseSelect ==2){
    ...
}

But I'm getting a compile error on this line that says:

Error: operand of '*' must be a pointer

Does this mean that the eraseSelect attribute should be a pointer as well as 'modePtr`? How would I fix this error?

Yury Schkatula · Accepted Answer · 2015-09-09 10:35:25Z

7

Try this:

if(modePtr->eraseSelect ==2){
    ...
}

or this:

if((*modePtr).eraseSelect ==2){
    ...
}

So you can use "dot" syntax to reach fields of an instance or "arrow" syntax to reach fields of a pointer to the instance. In most cases, "arrow" is more suitable.

edited Sep 9, 2015 at 10:35

answered Sep 9, 2015 at 9:47

Yury Schkatula

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Comments

marcinj · Accepted Answer · 2015-09-09 09:47:30Z

5

You dont need to dereference your pointer here:

if(modePtr->eraseSelect ==2){
    ...
}

answered Sep 9, 2015 at 9:47

marcinj

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Comments

therainmaker · Accepted Answer · 2015-09-09 09:46:50Z

5

The issue is at *modePtr->eraseSelect.

The -> is used to access member variables of pointers to objects. So, ptr->someMember is equivalent to *(ptr).someMember. You are mixing up the two, and so it doesn't make sense as you are dereferencing twice.

You should instead use modePtr->eraseSelect instead.

answered Sep 9, 2015 at 9:46

therainmaker

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