Is there any way of doing parallel assignment in C++? Currently, the below compiles (with warnings)
#include <iostream>
int main() {
int a = 4;
int b = 5;
a, b = b, a;
std::cout << "a: " << a << endl
<< "b: " << b << endl;
return 0;
}
and prints:
a: 4
b: 5
What I'd like it to print ... if it weren't obvious, is:
a: 5
b: 4
As in, say, ruby, or python.
That's not possible. Your code example
a, b = b, a;
is interpreted in the following way:
a, (b = b), a
It does nothing. The comma operator makes it return the value of a (the right most operand). Because assignment binds tighter, b = b is in parens.
The proper way doing this is just
std::swap(a, b);
Boost includes a tuple class with which you can do
tie(a, b) = make_tuple(b, a);
It internally creates a tuple of references to a and b, and then assigned to them a tuple of b and a.
Parallel assignment is not supported in C++. Languages that support this usually treat a,b,c as a list on either side of the assignment operator, this isn't the way the comma operator works in C++. In C++, a, b evaluates a and then b, so a, b = b, a is the same as a; b = b; a;.
Or Perl. But no, it's not possible (as far as I'm aware), you need to use a temporary variable, as in:
int a = 4;
int b = 5;
{
int tmp = a;
a = b;
b = tmp;
}
FYI, internally those languages (or Perl atleast) create a temporary list { a, b }, then assign the list to the two variables. In other words, this is atleast as performant, if only a little more messy.
This question is very old – I just stumbled into it today...
...and wondered why nobody gave this answer before...
I think, it's possible to do it in C++11 similar like Python does it (under the hood):
#include <iostream>
using namespace std;
int main()
{
int a = 4, b = 5;
cout << "Before assignment: a: " << a << ", b: " << b << endl;
pair<int&, int&> ba(b, a);
ba = make_pair(a, b); // <===: (b, a) = (a, b)
cout << "After assignment : a: " << a << ", b: " << b << endl;
return 0;
}
I tried this out on ideone.com. The output was:
Before assignment: a: 4, b: 5
After assignment : a: 5, b: 4
If I remember right (I'm not a Python expert), in Python, a, b denotes a pair. (Python Doc.: 5.3. Tuples and Sequences)
Such pair can be done in C++ 11 easily e.g. with std::pair. In this case, I made a pair of references and assigned the pair of values. It works as the make_pair() loads both variables before the right pair (of values) is assigned to the left pair of references.
Scrolling again, I realize that this answer is close to the boost based solution of Johannes answer.
May be, the reason is that it didn't work in C++03. I tried in coliru.stacked-crooked.com: With -std=c++03 it yields terrible to read compiler errors – changing to -std=c++11 and it compiles and executes fine as described above.
Disclaimer
I just cannot imagine what this solution is good for nor what practical worth it may have. This is not what I tried to do. As many other answers states "It does not work." IMHO it does (spelling it right according to the C++ language)...
Or Lua...
There are tricks with C/C++, like using xor or operations, but with risk of overflow and such. Just do it the painful way, with three assignments. Not a big deal.
There is no such function in the Standard Library. You could write a set of template functions :
template <typename T1> void ParAssign(T1& Lhs_1, T1 const& Rhs1);
template <typename T1, typename T2> void ParAssign(T1& Lhs1, T2& Lhs2, T1 const& Rhs1, T2 const& Rhs2);
// etc.
ParAssign(a,b,
b,a);
That's non-trivial if there is aliasing, as in your swap example.
