Error "no match for 'operator+" when concatenating strings - c++

Question

I am trying to reverse the words in a string using this code:
#include <bits/stdc++.h>
using namespace std;
int main()
{
//_ _ the sky is blue
string vec;
getline(cin, vec);
stack<string> str;
string temp = "";
string ans = "";
for (int i = 0; i < vec.length(); i++)
{
if (vec.at(i) == ' ')
{
if (temp.length() > 0)
{
str.push(temp);
temp = "";
}
else
{
temp = temp + vec.at(i);
}
}
}
//ans = ans + temp;
while (!str.empty())
{
ans = ans + " " + str.pop();
}
if (ans.length() != 0 && ans.at(0) == ' ')
ans = ans.substr(1);
cout << ans << endl;
}
I'm receiving this error at line 33 telling "no match for 'operator+'".
I have attached the relevant screenshot:
Please, help.

Answer 1

pop() is a stack member method with void return type, it doesn't return a string, therefore it cannot be printed neither can it be concatenated with other strings.
As the error shows you you can't add void and string using + operator for these 2 different types (unless you made that option available by overloading the + operator), so ans = ans + " " + str.pop(); is wrong.
You could use:
while (!str.empty())
{
ans = ans + " " + str.top();
str.pop();
}
As top() does return a string object.
I should point out that using #include <bits/stdc++.h> is bad and using namespace std is also not very good, but bringing them together is a disaster waiting to happen.

Answer 2

The method pop of the container adapter std::stack has the return type void. So this statement
ans= ans+" "+str.pop();
is incorrect and the compiler will issue an error.
You need to write something like
while(!str.empty()){
ans= ans+" "+ str.top();
str.pop();
}
Pay attention to that this for loop
for(int i=0 ; i<vec.length(); i++){
if(vec.at(i)==' '){
if(temp.length()>0){
str.push(temp);
temp = "";
}
else{
temp = temp + vec.at(i);
}
}
}
has a bug. If the string stored in the object vec does not end with a space character then the last word of the string will not be pushed on the stack.
It seems what you are trying to do is the following.
#include <iostream>
#include <string>
#include <stack>
#include <cctype>
int main()
{
std::string s( " Hello World " );
std::stack<std::string> st;
std::cout << "\"" << s << "\"\n";
for ( std::string::size_type i = 0; i < s.size(); )
{
std::string tmp;
while ( i < s.size() && isspace( ( unsigned char )s[i] ) )
{
tmp += s[i++];
}
if ( !tmp.empty() )
{
st.push( tmp );
tmp.clear();
}
while ( i < s.size() && !isspace( ( unsigned char )s[i] ) )
{
tmp += s[i++];
}
if ( !tmp.empty() )
{
st.push( tmp );
}
}
std::string result;
while ( !st.empty() )
{
result += st.top();
st.pop();
}
std::cout << "\"" << result << "\"\n";
return 0;
}
The program output is
" Hello World "
" World Hello "

Answer 3

Thank you guys for helping me out. Here's code and working perfectly fine:
#include <bits/stdc++.h>
using namespace std;
int main(){
//_ _ the sky is blue
string vec;
getline(cin, vec);
stack<string>str;
string rs;
string temp="";
string ans= "";
for(int i=0 ; i<vec.length(); i++){
if(vec.at(i)==' '){
if(temp.length()>0){
str.push(temp);
temp = "";
}
}
else{
temp = temp + vec.at(i);
}
}
ans = ans + temp;
while(!str.empty()){
ans= ans+" "+str.top();
str.pop();
}
if(ans.length() != 0 && ans.at(0) == ' '){
ans = ans.substr(1);
}
cout<<ans<<endl;
reverse( ans.begin(), ans.end());
cout<<ans<<endl;
}
Here is the output which only allows single space between each words and eliminates both leading and trailing space: