Is it a bad idea to define class variables in a header file?

// myHeader.hpp

class Foo {

public:
    Foo(SomeClassPtr *ptr, std::string text);
    Foo(std::string text);
    Foo();
    // some code where it is checked that scPtr is not a nullptr
    ~Foo();
private:
    SomeClassPtr *scPtr=nullptr;
    
    // some other members

}

I don't want to write scPtr=nullptr in the 2 Foo constructors that don't have the SomeClassPtr as an argument I just made the scPtr a nullptr by default. Now I want to know if this is a bad idea or if this ok to do i.e to define class variables in a header file.