C++ giving me basic_string::_S_construct null not valid error after I run my program

Question

I am a newbie to C++ and I've tried to write a simple string reverse program. When I compile it, everything is OK, but when I run it, I get the following error:

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

What I am doing wrong? Here is my code.

#include <iostream> using namespace std; string reverse_string(char* argv[], int i); int main(int argc, char* argv[]) { for (int i = 0; i < argc; i++) { cout << reverse_string(argv, i) << endl; } return 0; } string reverse_string(char* argv[], int i) { string arg = argv[i + 1]; string output; int length = arg.length(); for (int index = 1; index <= length; index++) { output += arg[length-index]; } return output; }

You are assigning a nullptr to string which is invalid, most likely it is argv[i+1]. See it live. — Shafik Yaghmour
– Shafik Yaghmour, Commented Feb 15, 2014 at 14:42
The error is because you are attempting to construct an std::string from a null char* at some point. — juanchopanza
– juanchopanza, Commented Feb 15, 2014 at 14:42
argv[argc], which happens on your last iteration, is a null pointer. — Qaz
– Qaz, Commented Feb 15, 2014 at 14:43

WhozCraig · Accepted Answer · 2014-02-15 14:50:00Z

This: argv[i + 1] in the construction of your reversed string should be argv[i] and the main loop should be for (i=1; i<argc; ++i)

And there are simpler ways to reverse a string:

std::string reverse_string(char* argv[], int i) { std::string arg = argv[i]; return std::string(arg.rbegin(), arg.rend()); }

Ok, I've noticed this already. And thanks for the simpler way :).

Vlad from Moscow · Accepted Answer · 2014-02-15 15:05:48Z

This error message

terminate called after throwing an instance of 'std::logic_error'
what(): basic_string::_S_construct null not valid

means that you are trying to call constructor of std::string passing as argument pointer NULL. The problem is that *(argv + argc) is NULL pointer.

Also take into account that you must include header <string>

As for the reverse function then it can be written much simpler then that of you. First of all it could have only one parameter of type const char *

For example

#include <iostream> #include <string> std::string reverse_string( const char* s ); int main(int argc, char* argv[]) { for (int i = 1; i < argc; i++) { std::cout << argv[i] << " -> " << reverse_string( argv[i] ) << std::endl; } return 0; } std::string reverse_string( const char* s ) { std::string arg( s ); return std::string( arg.rbegin(), arg.rend() ); }

One note: argv + argc cannot be a null pointer because argv[argc] is guaranteed to be a null pointer.
Yep, I understood that already. And about const char* s, I am still learning :). So thats why I din't use const char*

JadedTuna · Accepted Answer · 2014-02-15 14:49:00Z

Ok thanks for help everyone! I just didn't want to reverse argument 0, which is program's name. But I found another way around it. If anyone is interested, here is the fixed code:

#include <iostream> using namespace std; string reverse_string(char* argv[], int i); int main(int argc, char* argv[]) { for (int i = 1; i < argc; i++) { cout << argv[i] << " -> " << reverse_string(argv, i) << endl; } return 0; } string reverse_string(char* argv[], int i) { string arg = argv[i]; string output; int length = arg.length(); for (int index = 1; index <= length; index++) { output += arg[length-index]; } return output; }

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