gecko/mfbt/Util.h

/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * vim: set ts=8 sw=4 et tw=99 ft=cpp:
 *
 * ***** BEGIN LICENSE BLOCK *****
 * Version: MPL 1.1/GPL 2.0/LGPL 2.1
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 * 1.1 (the "License"); you may not use this file except in compliance with
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#ifndef mozilla_Util_h_
#define mozilla_Util_h_

#ifdef __cplusplus

namespace mozilla {

/**
 * DebugOnly contains a value of type T, but only in debug builds.  In
 * release builds, it does not contain a value.  This helper is
 * intended to be used along with ASSERT()-style macros, allowing one
 * to write
 *
 *   DebugOnly<bool> check = Func();
 *   ASSERT(check);
 *
 * more concisely than declaring |check| conditional on #ifdef DEBUG,
 * but also without allocating storage space for |check| in release
 * builds.
 *
 * DebugOnly instances can only be coerced to T in debug builds; in
 * release builds, they don't have a value so type coercion is not
 * well defined.
 */
template <typename T>
struct DebugOnly
{
#ifdef DEBUG
    T value;

    DebugOnly() {}
    DebugOnly(const T& other) : value(other) {}
    DebugOnly& operator=(const T& rhs) {
        value = rhs;
        return *this;
    }

    operator T&() { return value; }
    operator const T&() const { return value; }

#else
    DebugOnly() {}
    DebugOnly(const T&) {}
    DebugOnly& operator=(const T&) {}   
#endif

    /*
     * DebugOnly must always have a destructor or else it will
     * generate "unused variable" warnings, exactly what it's intended
     * to avoid!
     */
    ~DebugOnly() {}
};

} /* namespace mozilla */

#endif /* __cplusplus */

#endif  /* mozilla_Util_h_ */