将常量传递给C ++构造函数

Question

I have a class where the constructor receives an integer:

class One
{
public:
    One(int i)
    {
        foo(i);
    }

    void foo(int i)
    {
        // Do something with i
    }
};

The above compiles fine.

I have a second class that has a member of type One. Compiling this results in an error where I pass the int (the error is "expected a type specifier"):

class Two
{
public:
    One x(1);
};

I can, however, initialize the member if it is a pointer:

class Three
{
public:
    One *x = new One(1);
};

Can I initialize the class without using a pointer?

Thanks!

Answer 1

With:

class Two
{
public:
    One x(1);
};

and based on language rules, the compiler attempts to parse x as a member function returning an object of type One , but rather than seeing valid parameter-declarations which requires at least a list of 0 or more type-specifier s, it sees a non-type .

 class Three { public: One *x = new One(1); }; 

Can I initialize the class without using a pointer?

Yes, use the uniform-brace-initialization syntax for value-initialization :

class Three
{
public:
    One x{1};
};

Or copy-initialization :

class Three
{
public:
    One x = 1;  //Uses converting constructor, 
    //see http://en.cppreference.com/w/cpp/language/converting_constructor

    //or
    One x = One(your, multiple, arguments, here);
};

Or member-initializer-lists in the Constructor:

class Three
{
public:
    Three(...) : x(1) { ... }
    One x;
    ...
};

Answer 2

One x(1);

is parsed as a function declaration, that is why it expects a type in parentheses. You can use

One x = 1;

or

One x{1};

instead.