Pattern review C++: using structs with pure virtual methods in place of interfaces

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C++

I just started to learn C++, coming from a C# and Java background.
I like to use interfaces in my code. In C++ though they do not exist so instead I make structs with pure virtual methods.

My questions are:

1) From what I understand there is no difference between struct and class besides the default accessor, but it is common place to see struct being used when the class is of a smaller size. Therefore I decided as a convention to use struct when writing interfaces since the way struct is viewed by programmers more closely resembles an interface. Are there any other conventions on this issue or is my convention bad or could cause confusion in any way?

2) Since an interface only has pure virtual and provides no implementation (most of the times, default methods are now a thing in Java) I thought it would be logical to only make a header file for it, instead of making both a .cpp and a .h file. Since a .cpp file is supposed to have the implementation and the .h file is supposed to have the method prototypes. Again, are there any other conventions on this issue or is my convention bad or could cause confusion in any way?

Example:

ISayHello.h

struct IRender
{
    virtual void render() = 0;
}

ImplementationExample.cpp

class Sprite : public IRender
{
public:
    void render() override
    {
        //code goes here
    }
}

What I have tried:

NOTE: My intention is to get the opinion of those with more experience on how my pattern works, if it has problems, and if there is any other convention on issue that my convention is addressing. If you think the question is off topic I would appreciate if you could redirect me to a better place to ask my question. This question for example was marked as off topic on the Code Review Stack Exchange, though in my opinion asking for review on a pattern you in your code is not off topic.