Why is this syntax legal in C, but not C++?

dwhitney67 · 03-13-2009, 07:51 AM

On occasion I run across code similar to the following:

Code:

struct Foo
{
  int value1, value2, value3;
};

int main()
{
  struct Foo f = { .value1 = 10, .value3 = 20 };
}

This type of structure initialization is helpful when only certain field(s) of a structure need to be initialized, where other field(s) can be ignored.

Typically, one normally sees an initialization as follows:

Code:

...
  struct Foo f = { 10, 0, 20 };
...

Anyhow, why is the first approach supported in C, but with C++, the compiler spits out syntax errors?

johnsfine · 03-13-2009, 08:18 AM

I had no idea you could do that in C.

It sure would be a great feature in C++.

In more complicated cases, it can be much more important to initialize members (or map contents, etc.) by name.

I usually do this (quoted from memory, not retested, so I might have some detail wrong):

Code:

int main()
{
  struct f_initialized : public Foo {
     f_initialized() {
       Foo &x = *this;
       x.value1 = 10;
       x.value3 = 20; }
  } f;
}

Obviously, that is a lot of overhead just for initializing a couple members. But in many cases (such as class static variables) you really want the initialization done as part of construction of the object. Sometimes the limited syntax of the existing constructor or initialization list is too restrictive.

If the above code isn't obvious: It defines a class derived from Foo for the purposes of initializing this one object. Inside the class definition, it defines a constructor which does the desired initialization by member name. Then it defines f as a single instance of that class.

In some cases, there may be consequences from the fact that the type of f is f_initialized rather than Foo. But in most cases where I have used this idiom, that detail doesn't matter.

SciYro · 03-13-2009, 11:58 AM

Because C and C++ are diffrent languages. While people claim that C is a subset of C++, the fact is that this is wrong. Most commonly used C features and syntax are supported in C++, but not all, and it just so happens that you found a feature that C has that C++ does not have.

graemef · 03-13-2009, 06:49 PM

In C++ a struct is a class with all its members public by default. Because it is a class the rules are different from C which doesn't have a concept of classes. Use constructors to initialise the values.

Whilst you can use an initialiser list if you have no user-declared constructors, no private or protected non-static data members, no base classes, and no virtual functions. In which case the members will be initialised in order and if less values are in the initialiser list than there are members then the latter ones will be default initialised.