CODEWITHSUNDEEP
c++operatorsiostream
Mircea Ispas
Mircea Ispas

Reputation: 20780

C++ stream and operator>> precedence

I have the following code snippet:

std::stringstream stream("ABC 123 CBA 321");
std::unordered_map<std::string, int> map;
std::string key;

And I'm trying to read the data like this:

stream>>key>>map[key];

Which can be rewritten as:

operator>>(stream, key).operator>>(map[key]);

Considering both >> and . operators have left to right associativity, can you please explain why map[key] evaluates before key is read from the stream?

Printing the map:

for(auto& it : map)
{
    std::cout<<it.first<<" "<<it.second<<std::endl;
}

Results in:

 123
ABC 321

Yes, there is a space before 123

Upvotes: 0

Views: 369

Answers (1)

Benjamin Lindley
Benjamin Lindley

Reputation: 103703

Associativity is irrelevant here. Associativity determines which operands are bound to which operators. It does not determine in which order operand sub-expressions are evaluated. So when you say:

a >> b >> c;

Associativity dictates that the operands are grouped as (a >> b) >> c, but it does not dictate the order in which the expressions a, b and c are evaluated. Before C++17, that order was left unspecified by the standard: this means a, b and c could be evaluated in any order. Since C++17, the order is defined to be a, then b, then c.

In other words, a compiler conforming to C++17 would print:

CBA 321
ABC 123

Or the other way around, since the container is std::unordered_map, but not jumbled words like the output you show.

Upvotes: 2

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