How to fix 'std::logic_error' what(): basic_string::_M_construct null not valid error?

Question

I am trying to check whether an input string is alphanumeric or more uppercase or empty. If the input string is among the above-malfunctioned strings, I simply want to return false/0 otherwise work with rest of the program which is working fine. The chunk of my program which is given problem :

std::string myfunc(std::string input){
    std::string b="";

    if (!input.size()) return 0;
    for (int i = 0; i < input.size(); i++){

        if ( input[i] < 'a' || input[i] > 'z'|| isalpha(input[i]) || isupper(input[i]) ) return 0;
    }
    b = input;
    //just copy the input string for now.
    return b;
}

and I call this function as

int main(){
    std::string input="Somthing";
    std::cout << myfunc(input)<< std::endl;
    return  0;
}

getting the below error?

terminate called after throwing an instance of 'std::logic_error'
  what():  basic_string::_M_construct null not valid
Aborted (core dumped)

This program runs well without these two edge cases. I am not able to understand the error and find a fix to it? Any suggestions on what I am doing wrong?

maybe you want to return a std::optional when the function returns either a string or nothing — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Jan 29, 2019 at 20:46
yeah, I was looking of this type, since I wanna return both, string in the normal case, but integer if there a malfunctioned string is received! — Anu
– Anu, Commented Jan 29, 2019 at 20:48
Are you always changing it inside? And what do you mean by changing it? const& is better if you may not always require changing it or returning it. — Matthieu Brucher
– Matthieu Brucher, Commented Jan 29, 2019 at 20:56

Remy Lebeau · Accepted Answer · 2019-01-30 01:11:17Z

The problem is the two return 0; statements in your function. The function returns a std::string, which has no constructors that accept an int as input. But, it does have a constructor that accepts a const char * pointer, which 0 is implicitly convertible to. However, constructing a std::string with a null char * pointer is undefined behavior, and your implementation has chosen to throw a std::logic_error exception that you are not catching in your code.

In this case, I would simply return a blank string instead:

std::string myfunc(const std::string &input){
    if (input.empty()) return "";
    for (int i = 0; i < input.size(); ++i){
        char ch = input[i];
        if ( !((ch >= 'a' && ch <= 'z') || (ch >= '0' && ch <= '9')) ) return "";
    }
    return input;
}

The caller can then check if the return value is empty, if it wants to:

if (myfunc(input).empty())
    // error, do something
else
    // OK, do something else

Which would be better served with a function that returns a bool instead of a std::string:

bool isvalid(const std::string &input){
    if (input.empty()) return false;
    for (int i = 0; i < input.size(); ++i){
        char ch = input[i];
        if ( !((ch >= 'a' && ch <= 'z') || (ch >= '0' && ch <= '9')) ) return false;
    }
    return true;
}

// if you still needed this function for something...
std::string myfunc(const std::string &input){
    if (!isvalid(input)) return "";
    return input;
}

if (!isvalid(input))
    // error, do something
else
    // OK, do something else

john · Accepted Answer · 2019-01-29 20:40:22Z

0

If you want to return false (or true) then you should change the return type of your function to bool

bool myfunc(std::string input) {
^^^^

Secondly if you mean to return false then that's what you should return

if (!input.size()) return false;
                          ^^^^^

Returning 0 from a boolean function is not an error because 0 will automatically be converted to false, but clearly it's stylistically better to say what you mean.

answered Jan 29, 2019 at 20:40

john

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4 Comments

Anu Over a year ago

I do understand that, but my return type is std::string(the expected return from the function). Could you suggest what should be the return value for the edge cases if I want to keep the return type of the functions as the string.

john Over a year ago

You're asking to return either a bool or a string from your function? That is possible in some languages but in a strongly typed language like C++ it's difficult. Your best bet would be to split your function in two, an initial function that returns true or false, and then a second function that returns a string, but is only called if the first function returns true.

juanchopanza Over a year ago

@anu You could return std::optional<std::string>, or throw an exception where you would have returned false.

Remy Lebeau Over a year ago

@anu What is wrong with simply returning a blank string on failure? return ""; or return string();

Muhammad Ansar · Accepted Answer · 2022-06-23 11:58:42Z

0

I have faced this error.

The solution to this error is to use string as a stringstream --- Name you to want.Then replace name instead cout and return name.str();

like this

string to_string(){ stringstream ss; ss<< age << ","<< first_name <<","<<last_name <<","<< standard ; return ss.str();}

answered Jun 23, 2022 at 11:58

Muhammad Ansar

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