Convert Log macro to objective C string
Convert Log macro to objective C string
We have a C++ library with the below DebugLog macro which writes to a file
std::string var("some string");
DebugLog( << "Output" << var );
We want to use this in our Objective C code so that both the ObjC and C++ library are writing to the same log file.
In our existing ObjC we use a log macro which when used with format parameters produces no warning from compiler
LOG("Error description %s", [[error localizedDescription] UTF8String]);
However all usages without format parameters produce warnings eg
LOG("initialising")
The LOG macro is defined as shown below and is generating a warning 'Format string is not a string literal'. Anyone know how to improve the code to remove this warning.
#define LOG(fmt, ...) do
NSString *tmp =[NSString stringWithFormat:[NSString stringWithUTF8String:fmt], ##__VA_ARGS__];
DebugLog( << [ tmp UTF8String] );
while(0)
NSLog
sorry we can't modify the library
– tech74
Sep 2 at 7:11
If you use your macros passing an NSString into it like this:
LOG(@"format", params) then you probably don't need stringWithUTF8String in the code.– algrid
Sep 2 at 7:39
LOG(@"format", params)
stringWithUTF8String
We have thousands of these LOG statements so wanted to resolve it this way rather than attempting to change like that if possible
– tech74
Sep 2 at 7:50
Haven't done C++ is donkeys years, but almost all functions that take a format string will emit a warning if your try to pass a variable for that format. The danger is that the variable string may contain characters that get interpreted ("progress is 50% complete") may cause odd behavior or crashes. The "solution" is to pass the variable as a variable, so something like
"%s",tmp.UTF8String, in however you'd write that in the ugly-operator-overload-abused world of C++ these days.– James Bucanek
Sep 2 at 16:53
"%s",tmp.UTF8String
1 Answer
1
From your examples it appears you are using LOG with a C-string as the format string, on that basis your original NSLog version could be:
LOG
NSLog
#define LOG(fmt, ...) NSLog(@fmt, ##__VA_ARGS__);
provided fmt is always a literal string. Macros operate before lexical analysis (or at least effectively do so, dependent on implementation) – hence the term "pre-processor" for the part of the compiler which handles macros, conditional compilation, etc. Given this your macro example:
fmt
LOG("Error description %s", [[error localizedDescription] UTF8String]);
expands to:
NSLog(@"Error description %s", [[error localizedDescription] UTF8String]);
and the compiler sees the format string as an Objective-C string.
In a similar way you can rewrite your macro to use your C++ function as:
#define LOG(fmt, ...) DebugLog( [NSString stringWithFormat:@fmt, ##__VA_ARGS__].UTF8String );
HTH
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What C++ library are you using? You may be able to just figure out the file and keep
NSLog– Dair
Sep 2 at 0:12