Cout printing with array pointers - weird behavior - c++

I was playing around with pointers and got results I did not expect:
#include <iostream>
#include <vector>
int main() {
int arr[4] = { 1, 2, 3, 4 };
int* pArr = arr;
std::cout << "First number: " << *pArr << " at address: " << pArr;
pArr++;
std::cout << "\nSecond number: " << *pArr << " at address: " << pArr;
pArr++;
std::cout << "\nThird number: " << *pArr << " at address: " << pArr;
pArr++;
std::cout << "\nFourth number: " << *pArr << " at address: " << pArr;
int* pArr2 = arr;
std::cout << "\n"
<< *pArr2++ << "\n"
<< *pArr2++ << "\n"
<< *pArr2++ << "\n"
<< *pArr2++ << "\n";
/*
int* pArr2 = arr;
std::cout << "\n"
<< ++ * pArr2 << "\n"
<< * ++pArr2 << "\n";
*/
}
The two different results:
1 2 3 4 - as expected using the first method
4 3 2 1 - using cout with multiple arguments I do not know the proper name.
So my question is - why does this happen? Using multiple cout statements results in expected for me code, while using just 1 cout results in backwards solution.
As a side note, another confusing thing to me is that pre-increment results in all values being equal. In the commented bit of code, the result is 3 3, no matter the ++ placement with respect to the *.

This code:
std::cout << "\n" << *pArr2++ << "\n";
std::cout << "\n" << *pArr2++ << "\n";
has a well defined order of modifications to pArr and will print
1
2
But this code:
std::cout << "\n" << *pArr2++ << "\n" << *pArr2++ << "\n";
invokes undefined behavior before c++17, because there are multiple modifications to pArr2 that are unsequenced. The program has UB, so it could print anything.
From c++17, there is a sequence point between the modifications, and the above code is guaranteed to print:
1
2

Related

Move semantics in parameter passing

I have following test example:
#include <iostream>
#include <vector>
void foo (std::vector<int> value) {
std::cout << "value "
<< &value
<< " "
<< value.data()
<< " "
<< value.size()
<< std::endl;
}
void foo2 (std::vector<int>&& rvalure_ref) {
std::cout << "rvalue_ref "
<< &rvalure_ref
<< " "
<< rvalure_ref.data()
<< " "
<< rvalure_ref.size()
<< std::endl;
}
int main() {
std::vector<int> value(5, 0);
std::cout << "init "
<< &value
<< " "
<< value.data()
<< " "
<< value.size()
<< std::endl;
foo(std::move(value));
std::cout << "done "
<< &value
<< " "
<< value.data()
<< " "
<< value.size()
<< std::endl;
}
The result of the code above is:
init 0x7ffed27c6450 0x56480bc1eeb0 5
value 0x7ffed27c6470 0x56480bc1eeb0 5
done 0x7ffed27c6450 0 0
Looks great:
Now, move to:
#include <iostream>
#include <vector>
void foo (std::vector<int> value) {
std::cout << "value "
<< &value
<< " "
<< value.data()
<< " "
<< value.size()
<< std::endl;
}
void foo2 (std::vector<int>&& rvalure_ref) {
std::cout << "rvalue_ref "
<< &rvalure_ref
<< " "
<< rvalure_ref.data()
<< " "
<< rvalure_ref.size()
<< std::endl;
}
int main() {
std::vector<int> value(5, 0);
std::cout << "init "
<< &value
<< " "
<< value.data()
<< " "
<< value.size()
<< std::endl;
foo2(std::move(value));
std::cout << "done "
<< &value
<< " "
<< value.data()
<< " "
<< value.size()
<< std::endl;
}
The result is:
init 0x7ffccc93a5c0 0x56124b3a8eb0 5
rvalue_ref 0x7ffccc93a5c0 0x56124b3a8eb0 5
done 0x7ffccc93a5c0 0x56124b3a8eb0 5
My problem is:
For the 1st case, it is perfectly called by "move semantics", and as you see, the ownership of the vector has been transfered to the function parameter. Finally, at "done", the data is null to verify the the vector at main() no longer owns the vector.
Now to explicitly claim the parameter is "rvalue reference", as case 2. As you see, actually it is like "call by (l)reference".
How can I figure out it?

strange problem in a c++ program with pointers

I wrote this simple c++ program and I got some strange results that I don't understand (results are described in the line comments)
int arr[3] {1, 2, 3};
int* p{ nullptr };
p = arr;
std::cout << p[0] << " " << p[1] << " " << p[2]; // prints 1 2 3, OK
p = arr;
std::cout << *(p++) << " " << *(p++) << " " << *(p); // prints 2 1 3 ??
p = arr;
std::cout << *p << " " << *(++p) << " " << *(++p); // prints 3 3 3 ??
p = arr;
std::cout << *p << " "; ++p;
std::cout << *p << " "; ++p;
std::cout << *p; // prints 1 2 3, OK
it seems that the pointer increments along a std::cout concatenation don't work.
What's wrong in my idea?
I supposed it should have worked.
best
final edit: I was using c++14, I switched to c++20 and now it works properly
thank you everybody!
int* p{ nullptr };
std::cout << p[0] << " " << p[1] << " " << p[2];
This is Undefined Behavior, as you are dereferencing nullptr, p does not point at valid memory yet.
p = arr;
std::cout << p[0] << " " << p[1] << " " << p[2];
This is well-defined behavior. p points at valid memory, is always incremented before dereferenced, and is incremented in a deterministic and valid manner. This is the same as if you had written the following instead:
std::cout << *(p+0) << " " << *(p+1) << " " << *(p+2);
p = arr;
std::cout << *(p++) << " " << *(p++) << " " << *(p);
p = arr;
std::cout << *p << " " << *(++p) << " " << *(++p);
Both of these are Undefined Behavior prior to C++17, because the order in which chained operator<< calls are evaluated is not guaranteed in earlier versions, the compiler is free to evaluate them in whatever order it wants. This is no longer the case in C++17 onward.
p = arr;
std::cout << *p << " "; ++p;
std::cout << *p << " "; ++p;
std::cout << *p;
This is well-defined behavior. p points at valid memory, is always dereferenced before incremented, and is incremented in a deterministic and valid manner.

Using stringstream's ignore

I am trying to use a stringstream as a buffer but I am unable to update the underlying streambuf :
#include <iostream>
#include <sstream>
int main () {
std::stringstream ss(std::ios_base::app|std::ios_base::in|std::ios_base::out); //ostringstream gives the same output
ss << "foo";
std::cout << "position get: " << ss.tellg() << std::endl;
std::cout << "position put: " << ss.tellp() << std::endl;
ss << "b";
std::cout << "position get: " << ss.tellg() << std::endl;
std::cout << "position put: " << ss.tellp() << std::endl;
char c;
ss >> c;
std::cout << "position get: " << ss.tellg() << std::endl;
std::cout << "position put: " << ss.tellp() << std::endl;
ss.ignore(1);
std::cout << "position get: " << ss.tellg() << std::endl;
std::cout << "position put: " << ss.tellp() << std::endl;
std::cout << ss.str() << std::endl;
return 0;
}
yields:
position get: 0
position put: 3
position get: 0
position put: 4
position get: 1
position put: 4
position get: 2
position put: 4
foob
Is it possible to force a reallocation of the streambuf underlying object ? If not, is this reallocation automatic and in which circumstance is it triggered ?
I know I can use ss.str to change the underlying buffer. But it is a pain to use it to manually update the buffer to a substr's version.
Note: I am doing a school project and must compile to c++98, hence, if you have a solution which is comptabible, it would be much appreciated.

simple game, while loop and checking strings

I am a beginner at c++ and I want to create simple game. You have vector of strings, then you check if line input matched the right answer.
I want to generate random number 1 ,2 or 3. Then check if line matches correct answer and count the points.
I am probably missing something basic, yet I dont know what.
Problems:
Input line get correctly read on only first iterations
somehow points (tocke) jumps to 45763 after finishing.
At beginning time (cas) is sometimes 2.
Code:
#include <iostream>
#include <vector>
#include <stdlib.h>
#include <time.h>
#include <string>
int main() {
int runde;
int tocke;
int cas;
std::cout << "\n" << "Pravila igre:" << "\n" << "Za pravilen odgovor dobis 1 tocko, za napacnega zgubis 2!"<<
"\n" << "Stevilo zivljenj si izberes sama!"<< "\n" << "\n" ;
std::cout << "Izberi stevilo zivljenj!:" << "\n";
std::cin >> runde ;
std::vector<std::string> latin = {"carum carvi", "artemisia absiinthium","coriandrum sativum"};
std::vector<std::string> slovene = {"navadna kumina", "pravi pelin", "koriander"};
tocke << 0;
cas << 0;
do {
int ind;
cas << cas + 1;
std::cout << "Round " << cas <<"! Ladies and gentlemans, buckle your seatbelts!"<<"\n" << "\n" ;
ind = std::rand() % 3;
std::cout << "ime rastline: " << slovene[ind] << "\n";
std::cin.ignore();
std::string line;
getline(std::cin, line);
std::cout << "\n";
if (latin[ind] == line){
std::cout << "Pravlino! Tocka zate!" << "\n";
tocke << tocke + 1;
std::cout << "Tocke == " << tocke << "\n" << "Zivjenja == " << runde << "\n" << "Prezivete runde == " << cas << "\n"<< "\n";
}
else
{
std::cout << "Napaka! :D" << "\n";
std::cout << "Pravilen odgovor == " << latin[ind] << "\n";
-- runde ;
tocke << tocke - 2;
std::cout << "Tocke == " << tocke << "\n" << "Zivjenja == " << runde << "\n" << "Prezivete runde == " << cas << "\n"<< "\n";
}
}while(runde >= 0 );
std::cout << "\n"<<"Stevilo tock == " << tocke <<"\n" << "St. prezivetih rund == " << cas - 1
<< "\n" ;
}
You seem to have a misconception regarding operators. << is NOT assignment, use = instead. So tocke << 0; doesn't assign 0 to tocke, it does bitshifting (on an uninitialized variable), then discards the result. tocke stays uninitialized and this causes problems later.
Instead of this:
tocke << 0;
cas << 0;
Do this:
tocke = 0;
cas = 0;
Also instead of cas << cas + 1; do cas++ and instead of tocke << tocke - 2; do tocke -= 2;. To learn how the assignment operators work, you can read about them here. Last but not least, try to see if your compiler gives you any warnings, it should complain about using uninitialized values.

Separate statement to print each operand

I'm new to C++ and learning from the C++ Primer book. I would like to know how I would separate statement to print each operand form the following code:
#include <iostream>
int main()
{
std::cout << "Enter two numbers: " << std::endl;
int v1 = 0, v2 = 0;
std::cin >> v1 >> v2;
std::cout << "The numbers " << v1 << " and " << v2 << " multiplied are equal to " << v1 * v2 << std::endl;
}
I think I am meant to simplify the last statement but unsure how. If anyone would be able to help, it would be very helpful for my learning.
Just do std::cout << for each operand.
std::cout << "The numbers ";
std::cout << v1;
std::cout << " and ";
std::cout << v2;
std::cout << " multiplied are equal to "
std::cout << v1 * v2;
std::cout << endl;