c++ - What does the const keyword do in an operator definition? -

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i don't understand const keyword used in front of return type , after parameter list of operator definition. taken example book.

const char& operator [] (int num) const {     if (num < getlength())         return buffer[num]; }

the c++ const keyword means "something cannot change, or cannot delegate operations change onto other entities." refers specific variable: either arbitrary variable declaration, or implicitly this in member function.

the const before function name part of return type:

const char&

this reference const char, meaning not possible assign new value it:

foo[2] = 'q'; // error

the const @ end of function definition means "this function cannot change this object , cannot call non-const functions on any object." in other words, invoking function cannot change any state.

const char& operator [] (int num) const {   this->modifysomething(); // error   buffer.modifysomething(); // error   return buffer[num]; }

the goal of const-correctness big topic, short version being able guarantee immutable state immutable. helps thread safety , helps compiler optimize code.


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