C++ Error: 'atoi' is not a member of 'std' [duplicate]

Question

I've searched high and low but nothing answers this specific issue. I am doing a coding assignment that functions perfectly as I want it to. However, per our instructors directions, we are not allowed to included the entire standard library, only specific functions (such as string, cout, etc.)

If I include <stdlib.h> my code functions perfectly. If I remove that inclusion, I receive errors on these lines:

lid = atoi((input.substr(0, index1)).c_str());

error: 'atoi' was not declared in this scope

lgpa = atof((input.substr(index1 + 1, (index2 - index1 - 1))).c_str());

error: 'atof' was not declared in this scope

I have attempted to include these functions directly and receive the following errors:

lid = std::atoi((input.substr(0, index1)).c_str());

error: 'atoi' is not a member of 'std'

lgpa = std::atof((input.substr(index1 + 1, (index2 - index1 - 1))).c_str());

error: 'atof' is not a member of 'std'

Any constructive input would be very much appreciated.

Answer 1

atoi and atof are declared in the C standard library header.

• <stdlib.h> used as atoi and its counterpart

• <cstdlib> used as std::atoi

if you removed these headers and it will throw errors

error: 'atoi' was not declared in this scope

Even if you try using std:: , it will not work because they are not recognised, you didnt include proper headers.

For C++ 11 and later, we can use

1. std::stoi for converting strings to an integer.

2. std::stod or std::stof for converting strings to a floating-point number.

a simple example

#include <iostream>
#include <string>

int main() {
    // Input string with an integer and a floating-point number separated by a comma.
    std::string input = "123,3.14";
    size_t comma = input.find(',');
    
    // Convert substrings to int and double using std::stoi and std::stod.
    int lid = std::stoi(input.substr(0, comma));
    double lgpa = std::stod(input.substr(comma + 1));
    
    std::cout << "lid = " << lid << "\n";
    std::cout << "lgpa = " << lgpa << "\n";
    return 0;
}

We can also use strol for atoi and use strtod instead atof here is simple example using forward declaration , so that we do not have include <stdlib> and <cstdlib>:-

#include <iostream>
#include <string>

// Forward declaration for strtod.
extern "C" {
    double strtod(const char*, char**);
}

int main() {
    std::string input = "3.14 extra text";
    char* end_ptr = nullptr;
    
    // Convert the initial part of the string to a double.
    double value = strtod(input.c_str(), &end_ptr);
    
    std::cout << "Converted value: " << value << "\n";
    std::cout << "Remaining string: " << (end_ptr ? end_ptr : "none") << "\n";
    
    return 0;
}