Can I write this if statement with a variable declaration on one line? [duplicate]

Question

I was wondering if there was a way to put this on one line?

if (auto r = getGlobalObjectByName(word)) r->doSomething; // This works fine  if (!auto r = getGlobalObjectByName(word)) r->doSomething; // Says "expected an expression"  if (auto r = getGlobalObjectByName(word) == false) r->doSomething; // Also doesn't work. 

I also tried surrounding it with extra brackets, but that doesn't seem to work. I find this really convenient doing it on one line.

Answer 1

Since C++17 you can use an initializer if-statement:

if (auto r = getGlobalObjectByName(word); !r) r->doSomething; 

The semantics are:

if (init-statement; condition) statement 

The only difference from the "traditional" if-statement is the init-statement, which initializes a variable in the block scope, similar to for-loops.

Answer 2

If you have C++17, use the if (init statement; condition) form. If not, you have three choices:

• Stop trying to keep this all on one line. For example:

  auto r = getGlobalObjectByName(word); if (!r) r->doSomething(); 

• Use an else:

  if (auto r = getGlobalObjectByName(word)) {} else r->doSomething(); 

(Note that this requires that r is a smart pointer with very strange semantics for the operator bool() function. OTOH, I assume this is actual a short piece of example code, rather than your actual code).

I think I would only use the else form if it was really important to keep everything on one line (to preserve a tabular formatting of the code for example).