lambda to take `char` argument - c++

Question

How I should define lambda to take char from string iterator? In the code below lambda detect_bracket has problem with input parameter x.
I don't want to delete ALL brackets from the string, just at the beginning and at the end.
auto detect_bracket = [](char* x){ return(')' == x || '(' == x);};
this->str.erase(std::remove_if(str.begin(), str.begin(),
detect_bracket)
);
this->str.erase(std::remove_if(str.back(), str.back(),
detect_bracket)
);

Answer 1

You should take char as the parameter type of the lambda with std::remove_if, since the signature of the predicate function is supposed to check the element directly.
auto detect_bracket = [](char x){ return(')' == x || '(' == x);};
this->str.erase(std::remove_if(str.begin(), str.end(),
detect_bracket)
);
Note std::string::back() won't work with std::remove_if. It will return a char and std::remove_if expects a range expressed by iterator.
And str.begin(), str.begin() is just an empty range, if you just want to remove element at the begin and end, you could
auto detect_bracket = [](char x){ return(')' == x || '(' == x);};
if (!this->str.empty()) {
this->str.erase(std::remove_if(str.begin(), str.begin() + 1, detect_bracket), str.begin() + 1);
}
if (!this->str.empty()) {
this->str.erase(std::remove_if(str.end() - 1, str.end(), detect_bracket), str.end());
}
Note we need to specify the correct end iterator for std::string::erase, because std::remove_if will return an iterator even if it found nothing, and then the char will be erased wrongly.
LIVE

Answer 2

std::remove_if is a function with the following signature:
template< class ForwardIt, class UnaryPredicate >
ForwardIt remove_if( ForwardIt first, ForwardIt last, UnaryPredicate p );
p - unary predicate which returns ​true if the element should be removed.
The signature of the predicate function should be equivalent to the following:
bool pred(const Type &a);
The type Type must be such that an object of type ForwardIt can be
dereferenced and then implicitly converted to Type. ​
All you need is to change your function parameter from char* to char.

Answer 3

Multiple remove_if and erase calls are anyway modifying/invalidating the string. Why not simply create a new string, and conditionally assign source string from 0th location or 1st location? And then assign till the last or second last character, conditionally?
string target;
target.assign(source.begin() + skip_if_bracket_at_begin,
source.end() - skip_if_bracket_at_end);