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    <p>Complex logic and constructor do not always mix well, and there are strong proponents against doing heavy work in a constructor (with reasons).</p> <p>The cardinal rule is that the constructor should yield a fully usable object.</p> <pre><code>class Vector { public: Vector(): mSize(10), mData(new int[mSize]) {} private: size_t mSize; int mData[]; }; </code></pre> <p>It does not mean a fully initialized object, you may defer some initialization (think lazy) as long as the user does not have to think about it.</p> <pre><code>class Vector { public: Vector(): mSize(0), mData(0) {} // first call to access element should grab memory private: size_t mSize; int mData[]; }; </code></pre> <p>If there is heavy work to be done, you might choose to proceed with a builder method, that will do the heavy work prior to calling the constructor. For example, imagine retrieving settings from a database and building a setting object.</p> <pre><code>// in the constructor Setting::Setting() { // connect // retrieve settings // close connection (wait, you used RAII right ?) // initialize object } // Builder method Setting Setting::Build() { // connect // retrieve settings Setting setting; // initialize object return setting; } </code></pre> <p>This builder method is useful if postponing the construction of the object yields a significant benefit. From example if the objects grab a lot of memory, postponing the memory acquisition after tasks that are likely to fail may not be a bad idea.</p> <p>This builder method implies Private constructor and Public (or friend) Builder. Note that having a Private constructor imposes a number of restrictions on the usages that can be done of a class (cannot be stored in STL containers, for example) so you might need to merge in other patterns. Which is why this method should only be used in exceptional circumstances.</p> <p>You might wish to consider how to test such entities too, if you depend on an external thing (file / DB), think about Dependency Injection, it really helps with Unit Testing.</p>
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    1. USMatthieu M.
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    1. CO+1 for keeping simple mechanisms in constructors. But suggesting the use of RAII could be worth it also though not that simple.
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      1. USStephane Rolland
    2. COThe problem with calling init() methods on a lazy basis is that you have to make absolutely sure they get called before use.
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      1. USDavid Thornley
    3. CO@David: I agree, and the simpler the better. I suggested it mainly for completeness.
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      1. USMatthieu M.
 

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