c++ - undefined reference - C++ 中的 `a = -b;` 和 `a = -1*b;` 以及 `a = 0-b` 之间应该有区别吗？

Question

也许这在某个地方得到了回答，我只是不知道要搜索它的词。我试图了解我是否看到了错误，或者我是否不理解减号运算符。 header 中常量的赋值是否导致编译器在“-”上跳闸？

我的项目具有以下结构，包括生成的文件:

test├── src│   ├── TipCoordinate.cpp│   └── TipCoordinate.hpp└── UnitTest    ├── main.cpp    ├── main.o    ├── Makefile    ├── TipCoordinate.o    ├── UnitTest.pro    └── UnitTest.pro.user

I have a constant defined in my class header as follows:

class TipCoordinate{
public:
    TipCoordinate() {};
    ~TipCoordinate() {};
    void Z(const float z);
private:
    static const float sREFERENCE_SPHERE_RADIUS = 12;
};

我的类 cpp 文件中的以下代码将无法编译:

#include "TipCoordinate.hpp"
void TipCoordinate::Z(const float z){
    float z_origin_ = -sREFERENCE_SPHERE_RADIUS;
}

我收到错误 .../model/TipCoordinate.cpp:143: error: undefined reference to 'TipCoordinate::sREFERENCE_SPHERE_RADIUS'

但是，如果我按如下方式更改它，那么它会非常高兴:

#include "TipCoordinate.hpp"
void TipCoordinate::Z(const float z){
    float z_origin_ = -1*sREFERENCE_SPHERE_RADIUS;
}

或者，这也适用:

#include "TipCoordinate.hpp"
void TipCoordinate::Z(const float z){
    float z_origin_ = 0-sREFERENCE_SPHERE_RADIUS;
}

我认为 Adam Rosenfield 的回答解决了我的问题，但为了回应一些评论，我已将上述摘录替换为可重现问题的最小且完整的示例。正如一些人猜测的那样，我正在使用 g++。版本是 g++ (Ubuntu/Linaro 4.5.2-8ubuntu4) 4.5.2 似乎优化器受到了影响，因为我使用的是“Qt Creator 2.1.0，基于 Qt 4.7.2(32 位)，基于2011 年 3 月 11 日”。这是我的项目文件:

QT       -= gui
INCLUDEPATH += ../src
TEMPLATE = app
SOURCES += main.cpp \
    ../src/TipCoordinate.cpp
HEADERS += \
    ../src/TipCoordinate.hpp

我的主文件如下，但是将项目模板更改为 lib 并删除主文件会导致问题消失。

int main() { return 0; }

我怀疑 Qt Creator 很重要，因为除了更改模板之外，当我更改其他看似不相关的因素时，问题会消失或重新出现。出于这个原因，我在下面包含了 Qt Creator 生成的 Makefile:

#############################################################################
# Makefile for building: UnitTest
# Generated by qmake (2.01a) (Qt 4.7.3) on: Sun Oct 14 15:05:23 2012
# Project:  UnitTest.pro
# Template: app
# Command: /usr/local/Trolltech/QtEmbedded-4.7.3/bin/qmake -spec /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/qws/linux-x86-g++ CONFIG+=debug QMLJSDEBUGGER_PATH=/usr/share/qtcreator/qml/qmljsdebugger -o Makefile UnitTest.pro
#############################################################################

####### Compiler, tools and options

CC            = gcc
CXX           = g++
DEFINES       = -DQT_NETWORK_LIB -DQT_CORE_LIB -DQT_SHARED
CFLAGS        = -pipe -g -Wall -W -D_REENTRANT $(DEFINES)
CXXFLAGS      = -pipe -g -Wall -W -D_REENTRANT $(DEFINES)
INCPATH       = -I/usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/qws/linux-x86-g++ -I. -I/usr/local/Trolltech/QtEmbedded-4.7.3/include/QtCore -I/usr/local/Trolltech/QtEmbedded-4.7.3/include/QtNetwork -I/usr/local/Trolltech/QtEmbedded-4.7.3/include -I../src -I.
LINK          = g++
LFLAGS        = -Wl,-rpath,/usr/local/Trolltech/QtEmbedded-4.7.3/lib
LIBS          = $(SUBLIBS)  -L/usr/local/Trolltech/QtEmbedded-4.7.3/lib -lQtNetwork -L/usr/local/Trolltech/QtEmbedded-4.7.3/lib -lQtCore -lpthread 
AR            = ar cqs
RANLIB        = 
QMAKE         = /usr/local/Trolltech/QtEmbedded-4.7.3/bin/qmake
TAR           = tar -cf
COMPRESS      = gzip -9f
COPY          = cp -f
SED           = sed
COPY_FILE     = $(COPY)
COPY_DIR      = $(COPY) -r
STRIP         = strip
INSTALL_FILE  = install -m 644 -p
INSTALL_DIR   = $(COPY_DIR)
INSTALL_PROGRAM = install -m 755 -p
DEL_FILE      = rm -f
SYMLINK       = ln -f -s
DEL_DIR       = rmdir
MOVE          = mv -f
CHK_DIR_EXISTS= test -d
MKDIR         = mkdir -p

####### Output directory

OBJECTS_DIR   = ./

####### Files

SOURCES       = main.cpp \
        ../src/TipCoordinate.cpp 
OBJECTS       = main.o \
        TipCoordinate.o
DIST          = /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/common/g++.conf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/common/unix.conf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/common/linux.conf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/common/qws.conf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/qconfig.pri \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/modules/qt_webkit_version.pri \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/qt_functions.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/qt_config.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/exclusive_builds.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/default_pre.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/debug.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/default_post.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/warn_on.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/qt.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/unix/thread.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/moc.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/resources.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/uic.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/yacc.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/lex.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/include_source_dir.prf \
        UnitTest.pro
QMAKE_TARGET  = UnitTest
DESTDIR       = 
TARGET        = UnitTest

first: all
####### Implicit rules

.SUFFIXES: .o .c .cpp .cc .cxx .C

.cpp.o:
    $(CXX) -c $(CXXFLAGS) $(INCPATH) -o "$@" "$<"

.cc.o:
    $(CXX) -c $(CXXFLAGS) $(INCPATH) -o "$@" "$<"

.cxx.o:
    $(CXX) -c $(CXXFLAGS) $(INCPATH) -o "$@" "$<"

.C.o:
    $(CXX) -c $(CXXFLAGS) $(INCPATH) -o "$@" "$<"

.c.o:
    $(CC) -c $(CFLAGS) $(INCPATH) -o "$@" "$<"

####### Build rules

all: Makefile $(TARGET)

$(TARGET):  $(OBJECTS)  
    $(LINK) $(LFLAGS) -o $(TARGET) $(OBJECTS) $(OBJCOMP) $(LIBS)

Makefile: UnitTest.pro  /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/qws/linux-x86-g++/qmake.conf /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/common/g++.conf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/common/unix.conf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/common/linux.conf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/common/qws.conf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/qconfig.pri \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/modules/qt_webkit_version.pri \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/qt_functions.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/qt_config.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/exclusive_builds.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/default_pre.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/debug.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/default_post.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/warn_on.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/qt.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/unix/thread.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/moc.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/resources.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/uic.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/yacc.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/lex.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/include_source_dir.prf \
        /usr/local/Trolltech/QtEmbedded-4.7.3/lib/libQtNetwork.prl \
        /usr/local/Trolltech/QtEmbedded-4.7.3/lib/libQtCore.prl
    $(QMAKE) -spec /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/qws/linux-x86-g++ CONFIG+=debug QMLJSDEBUGGER_PATH=/usr/share/qtcreator/qml/qmljsdebugger -o Makefile UnitTest.pro
/usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/common/g++.conf:
/usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/common/unix.conf:
/usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/common/linux.conf:
/usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/common/qws.conf:
/usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/qconfig.pri:
/usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/modules/qt_webkit_version.pri:
/usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/qt_functions.prf:
/usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/qt_config.prf:
/usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/exclusive_builds.prf:
/usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/default_pre.prf:
/usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/debug.prf:
/usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/default_post.prf:
/usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/warn_on.prf:
/usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/qt.prf:
/usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/unix/thread.prf:
/usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/moc.prf:
/usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/resources.prf:
/usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/uic.prf:
/usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/yacc.prf:
/usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/lex.prf:
/usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/features/include_source_dir.prf:
/usr/local/Trolltech/QtEmbedded-4.7.3/lib/libQtNetwork.prl:
/usr/local/Trolltech/QtEmbedded-4.7.3/lib/libQtCore.prl:
qmake:  FORCE
    @$(QMAKE) -spec /usr/local/Trolltech/QtEmbedded-4.7.3/mkspecs/qws/linux-x86-g++ CONFIG+=debug QMLJSDEBUGGER_PATH=/usr/share/qtcreator/qml/qmljsdebugger -o Makefile UnitTest.pro

dist: 
    @$(CHK_DIR_EXISTS) .tmp/UnitTest1.0.0 || $(MKDIR) .tmp/UnitTest1.0.0 
    $(COPY_FILE) --parents $(SOURCES) $(DIST) .tmp/UnitTest1.0.0/ && $(COPY_FILE) --parents ../src/TipCoordinate.hpp .tmp/UnitTest1.0.0/ && $(COPY_FILE) --parents main.cpp ../src/TipCoordinate.cpp .tmp/UnitTest1.0.0/ && (cd `dirname .tmp/UnitTest1.0.0` && $(TAR) UnitTest1.0.0.tar UnitTest1.0.0 && $(COMPRESS) UnitTest1.0.0.tar) && $(MOVE) `dirname .tmp/UnitTest1.0.0`/UnitTest1.0.0.tar.gz . && $(DEL_FILE) -r .tmp/UnitTest1.0.0


clean:compiler_clean 
    -$(DEL_FILE) $(OBJECTS)
    -$(DEL_FILE) *~ core *.core


####### Sub-libraries

distclean: clean
    -$(DEL_FILE) $(TARGET) 
    -$(DEL_FILE) Makefile


check: first

mocclean: compiler_moc_header_clean compiler_moc_source_clean

mocables: compiler_moc_header_make_all compiler_moc_source_make_all

compiler_moc_header_make_all:
compiler_moc_header_clean:
compiler_rcc_make_all:
compiler_rcc_clean:
compiler_image_collection_make_all: qmake_image_collection.cpp
compiler_image_collection_clean:
    -$(DEL_FILE) qmake_image_collection.cpp
compiler_moc_source_make_all:
compiler_moc_source_clean:
compiler_uic_make_all:
compiler_uic_clean:
compiler_yacc_decl_make_all:
compiler_yacc_decl_clean:
compiler_yacc_impl_make_all:
compiler_yacc_impl_clean:
compiler_lex_make_all:
compiler_lex_clean:
compiler_clean: 

####### Compile

main.o: main.cpp 
    $(CXX) -c $(CXXFLAGS) $(INCPATH) -o main.o main.cpp

TipCoordinate.o: ../src/TipCoordinate.cpp ../src/TipCoordinate.hpp
    $(CXX) -c $(CXXFLAGS) $(INCPATH) -o TipCoordinate.o ../src/TipCoordinate.cpp

####### Install

install:   FORCE

uninstall:   FORCE

FORCE:

我看到一个滚动框出现了，但如果有办法让它看起来更简洁，我欢迎任何提示。

Answer 1

您看到的行为差异在于优化器。无论出于何种原因，优化器决定在某些情况下优化数学并用浮点常量替换它，但不是所有情况。当它这样做时，不会发生错误，因为变量 sREFERENCE_SPHERE_RADIUS 从未在任何地方被引用。

应该发生的是，在所有情况下您都应该得到“未定义引用”错误，因为需要为变量 sREFERENCE_SPHERE_RADIUS 提供显式类定义之外的定义。 C++ 标准只允许您为 static 类成员提供整数或枚举类型的初始化器:

// Header file
class TipCoordinate
{
    static const float sREFERENCE_SPHERE_RADIUS;  // declaration
};

// Source file
const float TipCoordinate::sREFERENCE_SPHERE_RADIUS = 12;  // definition

来自 C++03 §9.2/4:

A member-declarator can contain a constant-initializer only if it declares a static member (9.4) of const integral or const enumeration type, see 9.4.2.

在C++语言的最新版本C++11中，规则稍微宽松一些，可以在声明中定义浮点常量，但必须使用constexpr代替常量。