[expr.prim.lambda] lambdas and non-captured, unevaluated operands
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From: Paul Bibbings on 22 Apr 2010 22:16 In [expr.prim.lambda], ��5.1.2/11 it says: "If a lambda-expression has an associated capture-default and its compound-statement uses (3.2) this or a variable with automatic storage duration and the used entity is not explicitly captured, then the used entity is said to be implicitly captured." Later, in ��5.1.2/13: "A lambda-expression appearing in a default argument shall not implicitly or explicitly capture an entity." and it gives the following Example (reduced for relevance to the current post): void f2() { int i = 1; // ... void g5(int = ([]{ return sizeof i; })()); // OK } >From the above, my reading is that g5 is OK since the presence of i does not, in this context, constitute a 'use'. Thus, from [basic.def.odr], ��3.2/2: "An expression is potentially evaluated unless it is an unevaluated operand (Clause 5) or a subexpression thereof. A variable or non-overloaded function whose name appears as a potentially-evaluated expression is used ..." and, of course, this is the case for the operand to sizeof in the example above: [expr.sizeof] ��5.3.3/1: "The sizeof operator yields the number of bytes in the object representation of its operand. The operand is either an expression, which is an unevaluated operand (Clause 5), or a parenthesized type-id." What appears to be missing in [expr.prim.lambda] (unless it is me that is missing something) is some statement indicating how it happens - in the absence of explicit or implicit capture - that i is nevertheless `available' in the context of the compound-statement of the lambda-expression in g5, above; in effect, by what mechanism it is available outside of being captured. I am wanting to get a better sense of this since gcc-4.5.0 does not accept the above code, failing it with: 11:53:03 Paul Bibbings(a)JIJOU /cygdrive/d/CPPProjects/nano/gcc_bugs $cat _5_1_2_13.cpp // file: _5_1_2_13.cpp void f2() { int i = 1; // ... void g5(int = ([]{ return sizeof i; })()); } 11:53:06 Paul Bibbings(a)JIJOU /cygdrive/d/CPPProjects/nano/gcc_bugs $i686-pc-cygwin-gcc-4.5.0 -Wall -Wno-unused -std=c++0x -pedantic -c _5_1_2_13.cpp _5_1_2_13.cpp: In lambda function: _5_1_2_13.cpp:6:37: error: local variable ��i�� may not appear in this context _5_1_2_13.cpp:6:40: error: return-statement with a value, in function returning 'void' _5_1_2_13.cpp: In function ��void f2()��: _5_1_2_13.cpp:6:44: error: default argument for ��int <anonymous>�� has type ��void�� Regards Paul Bibbings -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |