Double Pointers in C/C++

Noah11012 on January 12, 2019

One source of confusion among new C programmers is pointers. At first glance, there doesn't seem to be much usefulness in using them. But, it is ... [Read Full]
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Probably worth noting as well, one of the more common uses:

int main(int argc, char** argv)

In this case, it's used as an array of char *, your program arguments.

 

Couldn't getline have returned a struct? Why is an output parameter necessary to begin with?

I think it's obvious I'm not a C guy, get over it ;-)

 

A struct is several bytes long, and you'd need to add in the cost of getline's return value as well. These values have to be pushed onto the stack, and then pulled back off and copied again into the receiving struct. That costs valuable processor cycles, but also it's a hidden cost that's not intuitive from reading the code, and therefore goes against C's design.

What might have been better is you have passed in a pointer to a struct, which would have involved fewer pushes and pops to stack - though with modern calling conventions the function arguments and return are in registers anyway, I think.

 
 

Maybe, but the POSIX people decided to use an output parameter.

 

That's not a maybe ;-)

I was asking about the design decision itself.

The return value of getline() is the number of characters read. Because this return value is already in use, they decided an output parameter for the contents would be sufficient.

A comment above made sense. It was in use doesn't justify the design.

A struct is several bytes long, and you'd need to add in the cost of getline's return value as well. These values have to be pushed onto the stack, and then pulled back off and copied again into the receiving struct. That costs valuable processor cycles, but also it's a hidden cost that's not intuitive from reading the code, and therefore goes against C's design.

 

It might be worth emphacising the value of typedefs to represent the semantics of the pointer "types". This reduces errors resulting from the confusion that your article alludes to by making double pointers look and act like single pointers. More a topic for the value of typedefs, but their usage can keep clear in one's mind, the usage and intent of the elements, pointer, double pointer, n-pointer, or no pointer.

 
 

Very well explained, remember learning them and It just blew me into oblivion. Would be interesting to see how someone could explain double pointers in the #explainlikeimfive way

 

Arrows and boxes, a good old fashioned diagram is what is called for. :)

I might take that challenge on another day.

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