Home/ Questions/How can i handle -basic_string::_m_construct null not valid?
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Adam Boulanger
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Asked: 2022-05-18T10:32:51+00:00 In:

How can i handle -basic_string::_m_construct null not valid?

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I am working on cpp, but I found the following warning message:

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

Is there any way to stabilize the issue “basic_string::_m_construct null not valid”? I read a lot of topics about this, but all of them were trying to install anything. Is this the correct way, or any recommendation for me? Please find the beginning command below:

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;
 }

int main(){
  std::string input="Somthing";
  std::cout << myfunc(input)<< std::endl;
  return 0;
 }
construct null not valid
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2 Answers

  1. Best Answer
    Expert

    The cause:

    There are two return 0; statements in your function. The function will return a std::string but there is no constructors get a int input. There is a constructor which can admit a const char * reference which 0 is changed implicitly. However, building a std::string with a null char * pointer is not a defined behavior. You don’t catch an exception std::logic_error which your implementation has selected in your code. Therefore, the error appears.

    Solution:

    To solve this problem effectively, you need to returan an empty string:

    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;
}

    Then the caller will check whether the return value has been set to empty or not:

    if (myfunc(input).empty())
//  incorrect, let's do something 
else
// correct, let's do something else

    This function returns a bool rather than a std::string, which would help you better.

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

if (!isvalid(input))
// incorrect, let's do something
else
// correct, let's do something else
    • 17

  2. You can return false or true if you change the return type to bool 

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

    If you want to return false, then you should do so. 

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

    It is not an error to return 0 from a Boolean Function. However, 0 will be automatically converted to false. But it is stylistically more important to clarify what you mean.

    • 17

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