remove_if last character from a string

Question

I would like to remove first and last brackets in passed by reference string. Unfortunately, I have difficulties with removing first and last elements conditionally. I cannot understand why remove_if doesn't work as I expect with iterators.

Demo

#include <iostream>
#include <algorithm>
using namespace std;

void print_wo_brackets(string& str){
    auto detect_bracket = [](char x){ return(')' == x || '(' == x);};
    if (!str.empty()) 
    {
        str.erase(std::remove_if(str.begin(), str.begin() + 1, detect_bracket));
    }
    if (!str.empty()) 
    {
        str.erase(std::remove_if(str.end()-1, str.end(), detect_bracket));
    }
}

int main()
{
    string str = "abc)";
    cout << str << endl;
    print_wo_brackets(str);
    cout << str << endl;


    string str2 = "(abc";
    cout << str2 << endl;
    print_wo_brackets(str2);
    cout << str2 << endl;

    return 0;
}

Output

abc)
ac    <- HERE I expect abc 
(abc 
abc

Answer 1

If remove_if returns end iterator then you will try to erase nonexistent element. You should use erase version for range in both places:

void print_wo_brackets(string& str){
    auto detect_bracket = [](char x){ return(')' == x || '(' == x);};
    if (!str.empty())
    {
        str.erase(std::remove_if(str.begin(), str.begin() + 1, detect_bracket), str.begin() + 1);
    }
    if (!str.empty())
    {
        str.erase(std::remove_if(str.end()-1, str.end(), detect_bracket), str.end());
    }
}

Answer 2

The problem is here:

if (!str.empty()) 
{
    str.erase(std::remove_if(str.begin(), str.begin() + 1, detect_bracket));
}

you erase unconditionally. std::remove_if returns iterator to the beginning of range "for removal". If there are no elements for removal, it returns end of range (str.begin() + 1 in this case). So you remove begin+1 element, which is b.

To protect from this problem you shouldn't probably do something more like:

if (!str.empty()) 
{
    auto it = std::remove_if(str.begin(), str.begin() + 1, detect_bracket);
    if(it != str.begin() + 1)
        str.erase(it);
}

I assume you simply want to check behavior of standard library and iterators, as otherwise check:

if(str[0] == '(' || str[0] == ')')
    str.erase(0);

is much simpler.