Warning “Implicit conversion loses integer precision…”

问题

I am doing a shopping cart tutorial: I have an array that collects input from a text field, and then displays it in the NSTableView. You can check an item, and remove it from the list. I want to display a warning only if something is checked. So, I have this:

    -(IBAction)removeItemFromShoppingList:(id)sender {
        int selectedItemIndex = [shoppingListTableView selectedRow];
        if (selectedItemIndex == -1) return;
        NSAlert *alert = [[NSAlert alloc] init];
        ...
        [alert runModal];
        [alert release];
}

On line 2 here (int selectedItemIndex...) I get a yellow warning: Implicit conversion loses integer precision:’NSInteger’ (aka ‘long’) to ‘int’.

Why?

回答1:

From apple's documentation:

When building 32-bit applications, NSInteger is a 32-bit integer. A 64-bit application treats NSInteger as a 64-bit integer.

#if __LP64__ || TARGET_OS_EMBEDDED || TARGET_OS_IPHONE || TARGET_OS_WIN32 || NS_BUILD_32_LIKE_64
typedef long NSInteger;
#else
typedef int NSInteger;
#endif

UPDATE:

Explain in detail:

[shoppingListTableView selectedRow] returns a NSInteger, and you are building an 64-bit application, so it is in fact a long.

You can use long selectedItemIndex instead of int selectedItemIndex to suppress this warning, but the warning appears again when building 32-bit version.

A better way is using NSInteger selectedItemIndex, which handles this case correctly.

回答2:

Because your variable is of type int and you are trying to copy the value of a variable of type NSInteger into it. An NSInteger can hold larger values than an int can, so you get a warning that overflow is possible. Probably the simplest fix is to change int to NSInteger. (When you want to copy the value of a variable to run tests on it, you should usually use a variable of the same type.)

回答3:

Use

int selectedItemIndex = (int)[shoppingListTableView selectedRow];

Instead of

int selectedItemIndex = [shoppingListTableView selectedRow];

回答4:

The selectedRow: method returns a value of type NSInteger.

You should make your selectedItemIndex of type NSInteger.

Please refer to:

http://developer.apple.com/library/mac/ipad/#documentation/Cocoa/Reference/ApplicationKit/Classes/NSTableView_Class/Reference/Reference.html

回答5:

Use

   NSInteger selectedItemIndex = [shoppingListTableView selectedRow];

Instead of

   int selectedItemIndex = [shoppingListTableView selectedRow];

回答6:

You can update project settings, to remove all

Implicit conversion loses integer precision warnings, by setting

Implicit Conversion to 32 Bit Type to No

in project's build settings.

回答7:

I get out warning just do:

int selectedItemIndex = (int)[shoppingListTableView selectedRow];

来源：https://stackoverflow.com/questions/8813799/warning-implicit-conversion-loses-integer-precision

标签

arrays

cocoa

warnings

nstableview