CODEWITHSUNDEEP
c++ternary-operator
Tanu Saxena
Tanu Saxena

Reputation: 779

Ternery operator in C++ strange behavior

General prototype: exp1?exp2:exp3

Ternary operator has a return type of exp2. It is necessary for exp3 to have same return type as exp2 or at least have an implicit conversion. Otherwise it will throw error

In the below program, I am getting an error in CodeBlocks as expected because of exp3 being int and exp2 being char*. Bjut when I am replacing 1 with 0, it is printing 0..

0 is also an int value.I am not able to understand.

 #include <iostream>
   using namespace std;

   int main()
   {
   int test = 0;
   cout << test ? "A String" : 1;

   return 0;
   }

Upvotes: 2

Views: 164

Answers (2)

Dietmar K&#252;hl
Dietmar K&#252;hl

Reputation: 153840

The ternary operator has rather low precedence. The only operator with lower precedence is the comma operator. Thus, your expression is interpreted as

(std::cout << test) ? "A String": 1;

This is probably not quite what you want. However, your question is actually about the difference between two expressions:

  1. the expression

    expr? "A String": 1

    is not working because there is no common type between a string literal, i.e., the type char const(&)[9], and an integer literal, i.e., the type int.

  2. the expression

    expr? "A String": 0

    is working because 0 can be considered to be a pointer constant and a string literal would happily decay to a char const*.

It is worth noting, as Chris Culter pointed out, that using a null pointer with the output operator is undefined behavior according to 27.7.3.6.4 [ostream.inserter.character] paragraph 3:

template<class traits>
    basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
                                          const char* s);

[...]

Requires: s shall not be a null pointer.

Upvotes: 8

ryanpattison
ryanpattison

Reputation: 6251

The stream operator has higher precedence so it will always output test. Try:

cout << (test ? "A String" : 1);

Upvotes: 2

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