Does C++ have "with" keyword like Pascal?

Question

with keyword in Pascal can be use to quick access the field of a record. Anybody knows if C++ has anything similar to that?

Ex: I have a pointer with many fields and i don't want to type like this:

if (pointer->field1) && (pointer->field2) && ... (pointer->fieldn)

what I really want is something like this in C++:

with (pointer)
{
  if (field1) && (field2) && .......(fieldn)
}

Solution 1

In C++, you can put code in a method of the class being reference by pointer. There you can directly reference the members without using the pointer. Make it inline and you pretty much get what you want.

Solution 2

Probably the closest you can get is this: (this is just an academic exercise. Of course, you can't use any local variables in the body of these artificial with blocks!)

struct Bar {
    int field;
};

void foo( Bar &b ) {
    struct withbar : Bar { void operator()() {
        cerr << field << endl;
    }}; static_cast<withbar&>(b)();
}

Or, a bit more demonically,

#define WITH(T) do { struct WITH : T { void operator()() {
#define ENDWITH(X) }}; static_cast<WITH&>((X))(); } while(0)

struct Bar {
    int field;
};

void foo( Bar &b ) {
    if ( 1+1 == 2 )
        WITH( Bar )
            cerr << field << endl;
        ENDWITH( b );
}

or in C++0x

#define WITH(X) do { auto P = &X; \
 struct WITH : typename decay< decltype(X) >::type { void operator()() {
#define ENDWITH }}; static_cast<WITH&>((*P))(); } while(0)

        WITH( b )
            cerr << field << endl;
        ENDWITH;

Solution 3

no there is no such keyword.

Solution 4

I like to use:

    #define BEGIN_WITH(x) { \
        auto &_ = x;

    #define END_WITH() }

Example:

    BEGIN_WITH(MyStructABC)
    _.a = 1;
    _.b = 2;
    _.c = 3;
    END_WITH()

Solution 5

Even though I program mostly in Delphi which has a with keyword (since Delphi is a Pascal derivative), I don't use with. As others have said: it saves a bit on typing, but reading is made harder.

In a case like the code below it might be tempting to use with:

cxGrid.DBTableView.ViewData.Records.FieldByName('foo').Value = 1;
cxGrid.DBTableView.ViewData.Records.FieldByName('bar').Value = 2;
cxGrid.DBTableView.ViewData.Records.FieldByName('baz').Value = 3;

Using with this looks like this

with cxGrid.DBTableView.ViewData.Records do
begin
  FieldByName('foo').Value = 1;
  FieldByName('bar').Value = 2;
  FieldByName('baz').Value = 3;
end;

I prefer to use a different technique by introducing an extra variable pointing to the same thing with would be pointing to. Like this:

var lRecords: TDataSet;

lRecords := cxGrid.DBTableView.ViewData.Records;

lRecords.FieldByName('foo').Value = 1;
lRecords.FieldByName('bar').Value = 2;
lRecords.FieldByName('baz').Value = 3;

This way there is no ambiguity, you save a bit on typing and the intent of the code is clearer than using with

View more solutions

Comments

[Ben Zotto]

Huh. Javascript has the with keyword and it does substantially the same thing; I didn't realize it had a pedigree reaching back to Pascal (!).

[Svetlozar Angelov]

Maybe it comes from Cobol or ADA, who knows...

[AProgrammer]

There is a with in Ada, but not with that meaning.

[Ignacio Soler Garcia]

There is a with in Vb and in Vb.Net with the same meaning too.

[erikkallen]

But in VB you need to say With obj / .X = x, so it's not as ambiguous

[Marco van de Voort]

There is always the question that if something can be abused, means that it is bad.

[Ankit Roy]

Sure, because name lookup in C++ is so straightforward. No -1, but I think the "can make code ambiguous" argument falls flat when you consider the case of argument-dependent lookup for functions, especially for instantiations of function templates.

[Ankit Roy]

This strategy works with C++ too, using a local T& or T const& variable to hold a reference to the long expression (which must be a proper lvalue in the T& case). To make it visually stand out, I usually name the variable _.

[Chris Walton]

Very nice :). I'd like to mention though that for those few of us that work with the Metrowerks compiler, this is quite likely not to work (it doesn't deal well with structs in a function)

[Potatoswatter]

@arke: really? That's surprising, I loved Metrowerks when it was the Mac standard. Hmm, it looks like they lost Howard Hinnant…

[karkael]

I agree it can be useful but I've seen it abused so many times now that I stay away from using it. In particular it can bite you if field1 is renamed in the record so your with-code now suddenly silently refers to another variable in scope with the same name. Instead of with I use one of the other approaches suggested here: use a local variable with short name or move it to a function/method.

[Nikola Gedelovski]

Upvote. The worst language feature. Consider this abomination: with struct1, struct2.substruct3, struct4.substruct5.ptrstruct6^... Seen it.

[Will]

Visual Basic tidied up the syntax by requiring the fields belonging to the 'with' to be prefixed with a "." - it also helped syntax completion etc. Our host Joel did that :)

[Dacav]

What do you mean for “make it inline”? Can you declare the method just inside another function? It's weird...

[Alexandre C.]

you can use static variables though.

[Steve Jessop]

Probably should be auto &p = *pointer, since your code takes a copy.

[Potatoswatter]

@Dacav This answer is just suggesting to add a method to the class being used by with. Being inline allows it to go into a header but otherwise that's a red herring. Such a method must be declared in its original class, and defined outside any other function. My answer below has a workaround to both problems.

[NicoBerrogorry]

This is actually sick good, because encapsulation is preserved... Thanks for making the connection!

[Server Overflow]

Not dangerous if you apply it only on a single type (record, structure, object, etc). In other words, never mix objects inside the "with" and your safe.

[Server Overflow]

it is NOT BY FAR the same!

[somebody4]

In c++17, it would be better to do if(auto a=foo();true){/**/}

[Sebastian]

This answer does not seem related to the question?