Clash Royale CLAN TAG#URR8PPP

C++ std::fill and std::fill_n error “error C2679”

What I'm trying to do:

I'm working in a code which is not mine and I'm trying to refactor this. There I have a struct type object, let's call it MY_STRUCTURE with some parameters inside. In the old version of that code, they are using memset to initialize the structure with 0 in all parameters. So, I've tried to change that memset to a more c++ standard approach which is fill or fill_n.

Also, I should mention... There is not an array, is just an object, so all that I'm trying to do is to initialize that object with 0 everywhere.

The memset approach which is working perfectly fine looks like

this:memset(&MyScruct, 0, sizeof(MyScruct));


What have I tried:

I'm trying to use one of fill and fill_n like this:

std::fill(&MyScruct, &MyScruct+ sizeof(MyScruct), 0);


and also

std::fill_n(&MyScruct, sizeof(MyScruct), 0);


where MyScruct is MY_STRUCT MyScruct;

MY_STRUCT MyScruct

In both of this cases I'm getting the same error:

error C2679: binary '=' : no operator found which takes a right-hand
operand of type 'const int' (or there is no acceptable conversion)

Do you have any idea why or what's happening here?

fill

fill_n

= 0;

fill

fill_n

MY_STRUCT

fill

*&MyScruct = 0;

2 Answers
2

In the original code, memset will completely ignore the type system and just write bytes to raw memory, trashing whatever was there before (and I have to question whether the "memset approach which is working perfectly fine" actually works fine, or is really undefined).

memset

On the other hand, std::fill works with the type system, and sets values of the right type. 0 is not a value of the right type (and is not assignable to the type), so you can't "fill" the struct with zeros.

std::fill

0

There is not an array, is just an object

Then you definitely don't want to pass sizeof(MyStruct) because that says how many objects of the type to "fill". You only have one object, not an array with sizeof(MyStruct) elements.

sizeof(MyStruct)

sizeof(MyStruct)

sizeof(MyStruct) is the right argument for memset because that works with bytes, not objects. You need to tell memset how many bytes to write. But it's totally wrong for std::fill and std::fill_n which count in objects, not bytes (unless by chance sizeof(MyStruct)==1, but even then it's a bug waiting to happen).

sizeof(MyStruct)

memset

memset

std::fill

std::fill_n

sizeof(MyStruct)==1

I've tried to change that memset to a more c++ standard approach which is fill or fill_n

std::fill and std::fill_n are the C++ way to assign new values to a sequence of values, not the C++ version of memset. The C++ version of memset is memset!

std::fill

std::fill_n

memset

memset

memset

If you're not trying to reset the values of the struct, just initialize it, then the correct way is just:

MyStruct s = ;


This is perfectly valid C++ and works in any version, from C++98 up to the latest standard. This is simpler and safer and more readable than using memset.

memset

If the code was written with memset correctly then there is no reason to change it. fill is not a substitute for memset.

memset

fill

memset

In modern C++ it would be bad to design a class to require memset. However if you are using someone else's class that was designed that way, it is correct to use their class as it was designed.

memset

In modern C++ you would design the class so that value-initialization resets it:

MyScruct = MY_STRUCT;


This code may or may not have the same effect as memset, depending on the details of MY_STRUCT.

memset

MY_STRUCT

MY_STRUCT

MyScruct = MY_STRUCT;

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