C++Guns – RoboBlog

29.05.2015

warning: conversion to 'float' from 'int' may alter its value [-Wconversion] (Bug of the day 7)

Filed under: Allgemein — Tags: , , — Thomas @ 22:05

Another bug of the day 😀

You can do the following

float var = float(vari); // yes I know I'm loosing precision
int var = int(std::floor(varf)) // yes I know interger can overflow
int var = qFloor(varf) // Qt version

Same for double.

warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

Tricky one. Usually don't use unsigned types. The world is full of negative value. Use signed types.
Except for..
Array index. To access the full 32/64bit address space, one must use unsigned type.

Let variable data be a pointer/array.
data[i] is the same as data+i
Now variable i should have the same wide as data. Which is size_t. Type size_t is 32bit wide on 32bit system and 64bit otherwise.
New rule: Always use size_t for index variable while accessing data. Don't use int.

Type size_t is an unsiged type.
New rule: Never count backwards in for loops while accessing data.

Consider this endless loop
for(size_t i=data.size(); i >= 0; i--)

If i == 0 and i-- then i == 2^32-1 which is > 0. Damn.
There are a lot of solutions. Bad one and really bad ones. Here is my perfect one: You count forward. That is really easy and everybody understands what is going one and there are no surprises.
In the next line you subtract n-i-1 and volia. Perfect.

for(size_t ri=0; ri < data.size(); ri++) { size_t i = data.size() - ri - 1; I named the new loop variabl ri for reverse iterator. Don't forget to add -1. Arrays start by 0. You can of couse use std::reverse_iterator But hey, I love for loops!

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