Primary expression error when defining a vector with ternary operator

Question

I have a simple code where I try to define a vector as one of two initializer lists using a ternary operator:

int main() { std::vector<int> const vec = true ? {3,4} : {5}; for (int const item : vec) { cout << item << endl; } return 0; }

But then I see the below primary-expression error:

tmp.cpp: In function ‘int main()’: tmp.cpp:7:41: error: expected primary-expression before ‘{’ token std::vector<int> const vec = true ? {3,4} : {5}; ^ tmp.cpp:7:41: error: expected ‘:’ before ‘{’ token tmp.cpp:7:41: error: expected primary-expression before ‘{’ token tmp.cpp:7:47: error: expected ‘,’ or ‘;’ before ‘:’ token std::vector<int> const vec = true ? {3,4} : {5};

I couldn't find anything relevant in either ternary operator or initializer list initialization. What am I missing here?

What are you really trying to do? Why are you using the ternary expression for this? Why can't you use plain assignment within a proper if ... else ...? — Some programmer dude
– Some programmer dude, Commented Jun 15, 2021 at 21:43
Try auto const vec = true ? std::vector<int>{3,4} : std::vector<int>{5}; — Remy Lebeau
– Remy Lebeau, Commented Jun 15, 2021 at 21:44
@Someprogrammerdude I was hoping for a one-liner code, didn't expect that line to throw up so many errors. Now I'm curious over what's happening — SJC
– SJC, Commented Jun 15, 2021 at 21:49

ShadowMitia · Accepted Answer · 2021-06-15 21:48:46Z

Because you can't have braces in that context. If you look at cppreference on list initialization, you see that the case inside a ternary operator isn't defined. So it can't be parsed correctly and you get the error you have.

You'd have to use something like this :

std::vector<int> const vec{(true ? std::vector<int>{3, 4} : std::vector<int>{5})};

Which basically expands to:

 if (true) { vec = std::vector<int>{3, 4}; } else { vec = std::vector<int> { 5 } };

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