While developing an application, I had the following problem. I wanted to return an empty std::list<string>
when a given function pointer was null, or the result of that function otherwise. This is a simplified version of my code:
typedef std::list<std::string> (*ParamGenerator)(); std::list<std::string> foo() { /* ... */ ParamGenerator generator = ...; if(generator) return generator(); else return {}; }
However, I usually like to use the ternary (?:
) operator in these cases, so I tried using it this way (as usual):
return generator ? generator() : {};
But got this error:
somefile.cpp:143:46: error: expected primary-expression before ‘{’ token somefile.cpp:143:46: error: expected ‘;’ before ‘{’ token
Does this mean I can't use the ternary operator to return objects created using their constructor from an initializer_list
? Is there any particular reason for that?
We use the ternary operator in C to run one code when the condition is true and another code when the condition is false. For example, (age >= 18) ? printf("Can Vote") : printf("Cannot Vote");
Nope, you can only assign values when doing ternary operations, not execute functions.
The conditional (ternary) operator is the only JavaScript operator that takes three operands: a condition followed by a question mark ( ? ), then an expression to execute if the condition is truthy followed by a colon ( : ), and finally the expression to execute if the condition is falsy.
Nested Ternary operator: Ternary operator can be nested. A nested ternary operator can have many forms like : a ? b : c.
Standard writes in 8.5.4.1: List-initialization
Note: List-initialization can be used
- as the initializer in a variable definition (8.5)
- as the initializer in a new expression (5.3.4)
- in a return statement (6.6.3)
- as a function argument (5.2.2)
- as a subscript (5.2.1)
- as an argument to a constructor invocation (8.5, 5.2.3)
- as an initializer for a non-static data member (9.2)
- in a mem-initializer (12.6.2)
- on the right-hand side of an assignment (5.17)
Nothing of them is a ternary operator. The more minimalistic return 1?{}:{};
is invalid too, what you want is impossible.
Of course you can explicitly call the constructor std::list<std::string>{}
, but I would recommend to write out the if
-else
-block as you already did.
When you do {}
the compiler has no knowledge of the type you are expecting, so it's just a meaningless expression that the compiler doesn't know what to do with. Both sides of the :
are evaluated separately, and only then will the compiler complain if the types don't match. I would just do this:
return generator ? generator() : std::list<std::string>();
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