𝐂++17 𝐜𝐨𝐩𝐲 𝐞𝐥𝐢𝐬𝐢𝐨𝐧 and 𝐭𝐞𝐦𝐩𝐨𝐫𝐚𝐫𝐲 𝐦𝐚𝐭𝐞𝐫𝐢𝐚𝐥𝐢𝐳𝐚𝐭𝐢𝐨𝐧

🎯 In this post, I’d like to discuss 𝐂++17 𝐜𝐨𝐩𝐲 𝐞𝐥𝐢𝐬𝐢𝐨𝐧 and 𝐭𝐞𝐦𝐩𝐨𝐫𝐚𝐫𝐲 𝐦𝐚𝐭𝐞𝐫𝐢𝐚𝐥𝐢𝐳𝐚𝐭𝐢𝐨𝐧 topics I find interesting!

🔗 𝐋𝐢𝐧𝐤 𝐭𝐨 𝐭𝐡𝐞 𝐞𝐱𝐚𝐦𝐩𝐥𝐞:
https://lnkd.in/gTwHw9Ef

Consider the code example that compiles in C++17 and later compilers, despite the deletion of copy and move constructors.
⛔ However, this code would not compile in 𝐩𝐫𝐞-𝐂++17 𝐜𝐨𝐦𝐩𝐢𝐥𝐞𝐫𝐬. Even though Return Value Optimization (𝐑𝐕𝐎) is applied, the presence of a copy or move constructor is still required.

🤔 𝐖𝐡𝐲 𝐢𝐬 𝐭𝐡𝐚𝐭 𝐭𝐡𝐞 𝐜𝐚𝐬𝐞?
🔎 To understand the underlying reason, it is essential to recognize how C++17 clarified the semantic meaning of one of the primary value categories: 𝐩𝐫𝐯𝐚𝐥𝐮𝐞𝐬("pure readable values")

➡ 𝐏𝐫𝐢𝐨𝐫 𝐭𝐨 𝐂++17,
Prior to C++17, prvalues (particularly of class type) always implied a temporary.
For instance in our example, 𝑿{} produced a temporary of type 𝘟.
➡ 𝐖𝐢𝐭𝐡 𝐂++17 :
prvalues are expressions for short-living values used 𝐟𝐨𝐫 𝐢𝐧𝐢𝐭𝐢𝐚𝐥𝐢𝐳𝐚𝐭𝐢𝐨𝐧𝐬.
For example the prvalue 𝑿{} itself does not produce a temporary. Instead, it initializes an object determined by the context: In our example, this object is referred to as 𝒙.

💡 So, when prvalues are used for initializations, 𝐧𝐨 𝐜𝐨𝐩𝐲 𝐨𝐫 𝐦𝐨𝐯𝐞 𝐨𝐩𝐞𝐫𝐚𝐭𝐢𝐨𝐧𝐬 𝐚𝐫𝐞 𝐞𝐯𝐞𝐫 𝐜𝐨𝐧𝐬𝐢𝐝𝐞𝐫𝐞𝐝. This is not merely an optimization; 𝘪𝘵 𝘪𝘴 𝘢 𝘨𝘶𝘢𝘳𝘢𝘯𝘵𝘦𝘦𝘥 𝘧𝘦𝘢𝘵𝘶𝘳𝘦 𝘰𝘧 𝘵𝘩𝘦 𝘭𝘢𝘯𝘨𝘶𝘢𝘨𝘦. As a result, we pass around the values of unmaterialized objects for initialization and this happens especially when a prvalue is passed to or returned from a function 𝒃𝒚 𝒗𝒂𝒍𝒖𝒆.

🤔So now what is 𝐭𝐞𝐦𝐩𝐨𝐫𝐚𝐫𝐲 𝐦𝐚𝐭𝐞𝐫𝐢𝐚𝐥𝐢𝐳𝐚𝐭𝐢𝐨𝐧?

🔄C++17 introduced the concept of materialization (of a temporary): the moment when a prvalue becomes a temporary object. This process is called 𝐚 𝐭𝐞𝐦𝐩𝐨𝐫𝐚𝐫𝐲 𝐦𝐚𝐭𝐞𝐫𝐢𝐚𝐥𝐢𝐳𝐚𝐭𝐢𝐨𝐧 𝐜𝐨𝐧𝐯𝐞𝐫𝐬𝐢𝐨𝐧, transforming a prvalue into an xvalue. Whenever a prvalue appears in a context that 𝐫𝐞𝐪𝐮𝐢𝐫𝐞𝐬 𝐚 𝐠𝐥𝐯𝐚𝐥𝐮𝐞 (such as an lvalue or xvalue), a temporary object is formed and initialized with the prvalue—since prvalues are primarily used for initializations in C++17. This conversion turns the prvalue into an xvalue, effectively representing the temporary.
𝐍𝐨𝐭𝐞 𝐭𝐡𝐚𝐭 the materialization process doesn't introduce extra copies or new objects.

I hope you found this discussion on a nuanced topic in C++ both informative and enjoyable! 😊 🌸

➡ 𝐑𝐞𝐬𝐨𝐮𝐫𝐜𝐞𝐬:
C++17 The Complete Guide 𝘕𝘪𝘤𝘰𝘭𝘢𝘪 𝘑𝘰𝘴𝘶𝘵𝘵𝘪𝘴
C++ Move Semantics The Complete Guide 𝘕𝘪𝘤𝘰𝘭𝘢𝘪 𝘑𝘰𝘴𝘶𝘵𝘵𝘪𝘴
C++ Templates The Complete Guide 2nd Edition 𝘕𝘪𝘤𝘰𝘭𝘢𝘪 𝘑𝘰𝘴𝘶𝘵𝘵𝘪𝘴, 𝘋𝘢𝘷𝘪𝘥 𝘝𝘢𝘯𝘥𝘦𝘷𝘰𝘰𝘳𝘥𝘦, 𝘋𝘰𝘶𝘨𝘭𝘢𝘴 𝘎𝘳𝘦𝘨𝘰𝘳

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