C++ variable not passing it's value after push_back

Question

I'm having a weird issue where the variables i'm declaring that are pushed back to a vector<vector<pixel>> are not being actually pushed back... Take a look:

vector<pixel> Lignetemp;
for(int j = 0; j < InfoSup.nbL; j++){
    Lignetemp.clear();
    for(int i = 0; i < InfoSup.nbC; i++){
        int Rtemp, Vtemp, Btemp;
        cin >> Rtemp;
        cin >> Vtemp;
        cin >> Btemp;
        Lignetemp.push_back({Rtemp,Vtemp,Btemp});
    }
    Data.push_back(Lignetemp);
}

Pixel is defined as such:

struct pixel{
    int R, V, B; 
};

The issue is that when I try to print out Data, I get only 0's instead of the values that I cin Also, I'm coding on both Windows and Linux ( g++ compilers of different versions) and namely on Ubuntu the compiler gives me these sort of warnings:

warning: extended initializer lists only available with -std=c++11 or -std=gnu++11

Do you have an idea about a fix ? Thanks :)

Looks like it should work. Can we get a compile-able [mcve]? — NathanOliver, Dec 04 '18 at 21:53
It should work, but as an aside, if you restricted the scope of `Lignetemp` to the inner loop, you wouldn't have to clear it. It is generally good practice to restrict scope as much as possible anyway. — Mike Borkland, Dec 04 '18 at 21:58
@NathanOliver prior to C++11 `v.push_back({a,b,c})` was a syntax error; in this case (the type being an aggregate) you had to do `T t(a, b, c); v.push_back(t);` — M.M, Dec 04 '18 at 22:00
@M.M Sure, but it compiling and running on windows so it must be doing something. The op just needs to turn on c++11 on their linux build. — NathanOliver, Dec 04 '18 at 22:02
@NathanOliver I've implemented those changes but nothing new ... Thank you however, this is getting frustrating — Falanpin, Dec 05 '18 at 07:52

Denis Malkin · Answer 1 · 2018-12-04T22:53:57.243

2

warning: extended initializer lists only available with -std=c++11 or -std=gnu++11

The warning gives you a clue.

You pass the initializer list to the push_back method. Here is it: {Rtemp,Vtemp,Btemp}

You can either:

Pass the flag, mentioned in the warning, to your compiler. How to pass it - depends on what is your build system.

Use explicit instantiation:

pixel p;
cin >> p.R >> p.V >> p.B;
Lignetemp.push_back(p);

edited Dec 04 '18 at 22:53

answered Dec 04 '18 at 22:03

Denis Malkin

21
4

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They could just do `cin >> p.R >> p.V >> p.B;` instead of having the temp ints – M.M Dec 04 '18 at 22:05

score 1 · Answer 2 · answered Dec 04 '18 at 22:03

1

Do as the warning suggests and add -std=c++11 or -std=gnu++11 to your compiler options.

The syntax Lignetemp.push_back({Rtemp,Vtemp,Btemp}); was not added to the language until C++11.

answered Dec 04 '18 at 22:03

M.M

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C++ variable not passing it's value after push_back

2 Answers2