-fpermissive option in codeblocks. Where do I put that “-fpermissive” option?

问题

#include <iostream>
#include <fstream>
#include <stdio.h>
#include <math.h>

I need help with this code. My compiler keeps asking me to use the -fpermissive option but I don't know where to input it. I've pasted the code below and the error that is shown.

using namespace std;

int cx = 0, cy = 0;

double x =0, y=0,  r = 5;


int main(){

ofstream myfile;

myfile.open("dingo.txt");

for(int g =0 ; g <= 360; g++)

//find x and y

//Get points of a circle.

x = cx + r * cos(g);

y = cy + r * sin(g);

//This is what pops up below:

//In function int main()':

//Error: name lookup of 'g' changed for ISO 'for' scoping [-fpermissive]

//note: (if you use '-fpermissive' G++ will accept your code)

//where should I enter the required option?

myfile << "X: " << x << "; Y: " << y <<endl;

myfile.close();

return 0;

}

回答1:

You can add more compiler flags under "Other Options" in "Settings" > "Compiler"

enter image description here

Although I think you should fix your code first. For example, std::sin and std::cos accepts radians, not degrees. You also need braces around your for statement.

for(int g =0 ; g <= 360; g++) {
    //code here.
}

回答2:

Don't use -fpermissive.

It means "I really, really know what I'm doing here, so please shut up" and is never, ever a good beginner's option.

And in this case, "g++ will accept your code" means "g++ will not complain about your code, but the bugs will still be there, and you will waste many hours looking for them since the code compiled without so much as a warning".

Indenting your code properly exposes the problem:

int main(){
    int cx = 0, cy = 0;
    double x = 0, y = 0, r = 5;
    ofstream myfile;
    myfile.open("dingo.txt");
    for(int g = 0 ; g <= 360; g++)
        x = cx + r * cos(g);
    y = cy + r * sin(g);  // <--- Here it is.
    myfile << "X: " << x << "; Y: " << y <<endl;
    myfile.close();
    return 0;
}

It's clear that the indicated line uses g, which is the loop variable.
In olden times, the scope of a variable declared in a for-loop was actually the scope enclosing the loop (the main function, in your case).
This was later changed so the scope of a loop variable is limited to the inside of the loop, but since there is a lot of legacy code that relies on the old rules, compilers provide a way to enable the obsolete behaviour.

What you intended was probably this:

for(int g = 0; g <= 360; g++)
{
    x = cx + r * cos(g);
    y = cy + r * sin(g);
    myfile << "X: " << x << "; Y: " << y <<endl;
}

(which is also wrong, because sin and cos use radians, not degrees - but I'll leave that problem as an exercise.)

来源：https://stackoverflow.com/questions/13024111/fpermissive-option-in-codeblocks-where-do-i-put-that-fpermissive-option

标签

c++

codeblocks