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Successful builds should be silent (warning-free). If they aren't, you'll quickly get into the habit of skimming the output, and you will miss real problems. (See Item 2.)
To get rid of a warning: a) understand it; and then b) rephrase your code to eliminate the warning and make it clearer to both humans and compilers that the code does what you intended.
Do this even when the program seemed to run correctly in the first place. Do this even when you are positive that the warning is benign. Even benign warnings can obscure later warnings pointing to real dangers.
#includes the original header and selectively turns
off the noisy warnings for that scope only, and then
#include your wrapper throughout the rest of your
project. Example (note that the warning control syntax will vary from
compiler to compiler):
// File: myproj/my_lambda.h -- wraps Boost's lambda.hpp // Always include this file; don't use lambda.hpp directly. // NOTE: Our build now automatically checks "grep lambda.hpp <srcfile>". // Boost.Lambda produces noisy compiler warnings that we know are innocuous. // When they fix it we'll remove the pragmas below, but this header will still exist. // #pragma warning(push) // disable for this header only #pragma warning(disable:4512) #pragma warning(disable:4180) #include <boost/lambda/lambda.hpp> #pragma warning(pop) // restore original warning level
Example 2: "Unused function parameter." Check to make sure you really didn't mean to use the function parameter (e.g., it might be a placeholder for future expansion, or a required part of a standardized signature that your code has no use for). If it's not needed, simply delete the name of a function parameter:
// ... inside a user-defined allocator that has no use for the hint ...
// warning: "unused parameter 'localityHint'"
pointer allocate( size_type numObjects, const void *localityHint = 0 ) {
return static_cast<pointer>( mallocShared( numObjects * sizeof(T) ) );
}
// new version: eliminates warning
pointer allocate( size_type numObjects, const void * /* localityHint */ = 0 ) {
return static_cast<pointer>( mallocShared( numObjects * sizeof(T) ) );
}
Example 3: "Variable defined but never used." Check to make sure you really didn't mean to reference the variable. (An RAII stack-based object often causes this warning spuriously; see Item 13.) If it's not needed, often you can silence the compiler by inserting an evaluation of the variable itself as an expression (this evaluation won't impact run-time speed):
// warning: "variable 'lock' is defined but never used"
void Fun() {
Lock lock;
// ...
}
// new version: probably eliminates warning
void Fun() {
Lock lock;
lock;
// ...
}
Example 4: "Variable may be used without being initialized." Initialize the variable (see Item 19).
Example 5: "Missing return." Sometimes the
compiler asks for a return statement even though your
control flow can never reach the end of the function (e.g., infinite loop,
throw statements, other returns).
This can be a good thing, because sometimes you only think that control
can't run off the end. For example, switch statements
that do not have a default are not resilient to change
and should have a default case that does
assert( false ) (see also Items 68 and 90):
// warning: missing "return"
int Fun( Color c ) {
switch( c ) {
case Red: return 2;
case Green: return 0;
case Blue:
case Black: return 1;
}
}
// new version: eliminates warning
int Fun( Color c ) {
switch( c ) {
case Red: return 2;
case Green: return 0;
case Blue:
case Black: return 1;
default: assert( !"should never get here!" ); // !"string" evaluates to false
return -1;
}
}
Example 6: "Signed/unsigned mismatch." It is usually not necessary to compare or assign integers with different signedness. Change the types of the variables being compared so that the types agree. In the worst case, insert an explicit cast. (The compiler inserts that cast for you anyway, and warns you about doing it, so you're better off putting it out in the open.)
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