What does the mean "[](unsigned char x)"?

Question

What does the mean [](unsigned char x)

Here is my code:

#include <algorithm>

std::string str1 = "Text with some     spaces";

str1.erase(std::remove(str1.begin(), str1.end(), ' '), str1.end());

std::cout << str1 << '\n';

std::string str2 = "Text\n with\tsome \t whitesspaces\n\n";

str2.erase(std::remove_if(str2.begin(), str2.end(), [](unsigned char x) {return std::isspace(x);}), str2.end());

std::cout << str2 <<'\n';

Answer 1

It's the beginning of a lambda function defintion:

[](unsigned char x) {
    return std::isspace(x);
}

This defines a temporary function receiving a unsigned char and returning an int (automatically determined by the return value of std::isspace, since the lambda did not specify a return type).