#include <iostream>
#include <string>

using namespace std;

class Test {
private:
	string str;
public:
	void Setstr(const string &st);
	void Getstr(string &st);
};

void Test::Setstr(const string &st) {
	this->str = st;
}

void Test::Getstr(string &st) {
	st = this->str;
}

int main() {
	string s;
	Test t();
	t.Setstr("Test");
	t.Getstr(s);
	
	cout << s << endl;
	return 0;
}

$ g++ -Wall -o test test.cpp
test.cpp: In function ‘int main()’:
test.cpp:25:4: error: request for member ‘Setstr’ in ‘t’, which is of non-class type ‘Test()’
  t.Setstr("Test");
    ^
test.cpp:26:4: error: request for member ‘Getstr’ in ‘t’, which is of non-class type ‘Test()’
  t.Getstr(s);
    ^

Or use "Test t{}". That gives uniformity to how variables are declared and initialized.