CODEWITHSUNDEEP
c++c++11stdstring
user526427
user526427

Reputation: 107

C++ string question - how to access string elements?

I have this simple code in C++:

#include <iostream>
#include <string>

using namespace std;

int main() {
    string s;

    s[0]='A';
    cout << "s is: " << s << endl
         << "s[0] is: " << s[0] << endl;
}

why is s empty but s[0] has value of 'A'? can C++ strings be assigned values to its elements directly via index? Thanks!

Upvotes: 0

Views: 635

Answers (2)

Remy Lebeau
Remy Lebeau

Reputation: 596121

why is s empty

Because you default-constructed it, so it is initially empty, and you did not do anything to allocate memory for its character storage.

but s[0] has value of 'A'?

It does not. You are assigning 'A' to memory that has not been allocated yet.

can C++ strings be assigned values to its elements directly via index?

Yes, but only for valid indexes in the range of 0..size()-1 when size() is > 0. In your example, index 0 is not valid because s.size() is 0.

#include <iostream>
#include <string>

using namespace std;

int main() {
    string s;

    s.resize(1); // <-- add this!

    s[0]='A';
    cout << "s is: " << s << endl
         << "s[0] is: " << s[0] << endl;
}

Upvotes: 1

Thomas Sablik
Thomas Sablik

Reputation: 16453

This is undefined behavior. The string s is empty. Until C++11 you can't access s[0]. Since C++11 you can read s[0] but you can't write it.

You could resize the string with

#include <iostream>
#include <string>

using namespace std;

int main() {
    string s;
    s.resize(1);

    s[0]='A';
    cout << "s is: " << s << endl
         << "s[0] is: " << s[0] << endl;
}

Read https://en.cppreference.com/w/cpp/string/basic_string/operator_at

Upvotes: 4

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