/Zc:throwingNew (Assume operator new throws)
When the /Zc:throwingNew option is specified, the compiler optimizes calls to
operator new to skip checks for a null pointer return. This option tells the compiler to assume that all linked implementations of
operator new and custom allocators conform to the C++ standard and throw on allocation failure. By default in Visual Studio, the compiler pessimistically generates null checks (/Zc:throwingNew-) for these calls, because users can link with a non-throwing implementation of
operator new or write custom allocator routines that return null pointers.
Since ISO C++98, the standard has specified that the default operator new throws
std::bad_alloc when memory allocation fails. Versions of Visual C++ up to Visual Studio 6.0 returned a null pointer on an allocation failure. Beginning in Visual Studio 2002,
operator new conforms to the standard and throws on failure. To support code that uses the older allocation style, Visual Studio provides a linkable implementation of
operator new in nothrownew.obj that returns a null pointer on failure. By default, the compiler also generates defensive null checks to prevent these older-style allocators from causing an immediate crash on failure. The /Zc:throwingNew option tells the compiler to leave out these null checks, on the assumption that all linked memory allocators conform to the standard. This does not apply to explicit non-throwing
operator new overloads, which are declared by using an additional parameter of type
std::nothrow_t and have an explicit
Conceptually, to create an object on the free store, the compiler generates code to allocate its memory and then to invoke its constructor to initialize the memory. Because the MSVC compiler normally cannot tell if this code will be linked to a non-conforming, non-throwing allocator, by default it also generates a null check before calling the constructor. This prevents a null pointer dereference in the constructor call if a non-throwing allocation fails. In most cases, these checks are unnecessary, because the default
operator new allocators throw instead of returning null pointers. The checks also have unfortunate side effects. They bloat the code size, they flood the branch predictor, and they inhibit other useful compiler optimizations such as devirtualization or const propagation out of the initialized object. The checks exist only to support code that links to nothrownew.obj or has custom non-conforming
operator new implementations. If you do not use non-conforming
operator new, we recommend you use /Zc:throwingNew to optimize your code.
The /Zc:throwingNew option is off by default, and is not affected by the /permissive- option.
If you compile by using link-time code generation (LTCG), you do not need to specify /Zc:throwingNew. When your code is compiled by using LTCG, the compiler can detect if the default, conforming
operator new implementation is used. If so, the compiler leaves out the null checks automatically. The linker looks for the /ThrowingNew flag to tell if the implementation of
operator new is conforming. You can specify this flag to the linker by including this directive in the source for your custom operator new implementation:
#pragma comment(linker, "/ThrowingNew")
For more information about conformance issues in Visual C++, see Nonstandard Behavior.
To set this compiler option in the Visual Studio development environment
Open the project's Property Pages dialog box. For details, see Set C++ compiler and build properties in Visual Studio.
From the Configuration drop down menu, choose All Configurations.
Select the Configuration Properties > C/C++ > Command Line property page.
Modify the Additional Options property to include /Zc:throwingNew or /Zc:throwingNew- and then choose OK.
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