错误：ISO C ++禁止无类型的声明

Question

I got error: ISO C++ forbids declaration of 'InvalidSig' with no type of the below header file, how to solve it?

struct args{
    InvalidSig* context; //error
    string mname;
};

class InvalidSig{
    .......
}

Answer 1

You can forward declare class InvalidSig above:

class InvalidSig;

struct args {
    InvalidSig* context;
    // ...
};

You can also do this, if you only want to use the name once before its definition:

struct args {
    class InvalidSig* context;
    // ...
};

I recommend avoiding the latter though, as the former is more common and will be less confusing to anyone reading your code.