为什么 setw 不显示空格？

Question

I have this code. It works for the first space, but it doesn't work for the second one. Can anyone tell me why, or how to fix it?

#include <iostream>
#include <iomanip>
#include <vector>
#include <string>
#include <fstream>

using namespace std;

//Constants
const int LENGTH_SPACE = 10;
const int LENGTH_SPACES = 2;

//Structs
struct post_t
{
    int authorID;
    int targetID;
    string message;
};

void displayStruct(post_t &demo)
{
    cout << demo.authorID << setw(LENGTH_SPACE) << demo.targetID << setw(LENGTH_SPACES) << demo.message << endl;
};

int main() {
    post_t demo = {101, 9999, "Just a test"};
    displayStruct(demo);

    return 0;
}

EXPECTED OUTPUT

101     9999  Just a test

Answer 1

You seem to expect setw(LENGTH_SPACE) to simply add spaces in between the other output.

For that you would not need setw (see below). What it does instead ( cppreference ):

When used in an expression out << setw(n) or in >> setw(n), sets the width parameter of the stream out or in to exactly n.

For example this:

#include <iostream>
#include <iomanip>
int main() {
    std::cout << std::setw(5) << "123" << "\n";
    std::cout << "^^ two spaces added to a 3 characters long output\n";
}

produces output:

  123
^^ two spaces added to a 3 characters long output

And output of this:

#include <iostream>
#include <iomanip>
int main() {
    std::cout << std::setw(2) << "123" << "\n";
    std::cout << "no space added when the length of the string is longer than the requested width\n";
}

is

123
no space added when the length of the string is longer than the requested width

---

If you simply want a certain number of spaces ' ' in between the output of the members of the struct, thats not what setw does. Instead:

std::cout << "a" << std::string(number_of_spaces,' ') << "b";