我有以下类，当我运行测试代码来测试operator /时，编译器会告诉我以下内容（指的是在除法运算符函数中进行的比较）
C2678: binary '==': no operator found which takes a left-hand operand of type 'const foo<8>' (or there is no acceptable conversion)
template<unsigned int t>
class foo {
foo(int i) {
// implicit conversion
}
template<unsigned int u>
bool operator==(const foo<u>& obj) const {
// do some comparison here
}
template<unsigned int u>
foo operator/(const foo<u>& divisor) const {
if (divisor == 0)
// perform division
// return foo result
}
};
但是，当我将以下代码添加到类中时，没有错误：
friend bool operator==(const foo<t>& lhs, const foo<t>& rhs) {
// do some comparison here
}
我的测试代码如下：
int main() {
foo<8> foo_a(0);
foo<4> foo_b(123);
foo_b / foo_a;
return 0;
}
答案 0 :(得分：1)
问题是在
中找到==
if (divisor == 0)
如果你只有
template<unsigned int u>
bool operator==(const foo<u>& obj) const {
// do some comparison here
}
编译器无法确定u的任何值是否会为foo<u>提供可从0转换的friend bool operator==(const foo<t>& lhs, const foo<t>& rhs) {
// do some comparison here
}
。所以它没有使用那个运算符。
另一方面，如果你有
lhs可以看出，如果foo<t>为rhs，那么foo(int i)也必须是foo<t>。然后，它可以使用构造函数0从值==构造elasticsearch (1.0.12)
elasticsearch-api (= 1.0.12)
elasticsearch-transport (= 1.0.12)
elasticsearch-api (1.0.12)
multi_json
elasticsearch-model (0.1.7)
activesupport (> 3)
elasticsearch (> 0.4)
hashie
elasticsearch-rails (0.1.7)
elasticsearch-transport (1.0.12)
。现在它具有 GIT
specs:
activeadmin (1.0.0.pre4)
arbre (~> 1.0, >= 1.0.2)
bourbon
coffee-rails
formtastic (~> 3.1)
formtastic_i18n
inherited_resources (~> 1.6)
jquery-rails
jquery-ui-rails
kaminari (~> 0.15)
railties (>= 3.2, < 5.1)
ransack (~> 1.3)
sass-rails
sprockets (< 4.1)
GEM
specs:
CFPropertyList (2.3.1)
actionmailer (4.2.7.1)
actionpack (= 4.2.7.1)
actionview (= 4.2.7.1)
activejob (= 4.2.7.1)
mail (~> 2.5, >= 2.5.4)
rails-dom-testing (~> 1.0, >= 1.0.5)
actionpack (4.2.7.1)
actionview (= 4.2.7.1)
activesupport (= 4.2.7.1)
rack (~> 1.6)
rack-test (~> 0.6.2)
rails-dom-testing (~> 1.0, >= 1.0.5)
rails-html-sanitizer (~> 1.0, >= 1.0.2)
actionview (4.2.7.1)
activesupport (= 4.2.7.1)
builder (~> 3.1)
erubis (~> 2.7.0)
rails-dom-testing (~> 1.0, >= 1.0.5)
rails-html-sanitizer (~> 1.0, >= 1.0.2)
active_link_to (1.0.2)
actionpack
active_model_serializers (0.10.2)
actionpack (>= 4.1, < 6)
activemodel (>= 4.1, < 6)
jsonapi (~> 0.1.1.beta2)
railties (>= 4.1, < 6)
activejob (4.2.7.1)
activesupport (= 4.2.7.1)
globalid (>= 0.3.0)
activemodel (4.2.7.1)
activesupport (= 4.2.7.1)
builder (~> 3.1)
activerecord (4.2.7.1)
activemodel (= 4.2.7.1)
activesupport (= 4.2.7.1)
arel (~> 6.0)
activerecord-postgres-earthdistance (0.4.2)
pg
rails (>= 3.1)
rake
activesupport (4.2.7.1)
i18n (~> 0.7)
json (~> 1.7, >= 1.7.7)
minitest (~> 5.1)
thread_safe (~> 0.3, >= 0.3.4)
tzinfo (~> 1.1)
addressable (2.3.8)
arbre (1.1.1)
activesupport (>= 3.0.0)
arel (6.0.3)
ast (2.3.0)
autoprefixer-rails (6.4.1)
execjs
awesome_print (1.6.1)
babel-source (5.8.35)
babel-transpiler (0.7.0)
babel-source (>= 4.0, < 6)
execjs (~> 2.0)
bcrypt (3.1.10)
binding_of_caller (0.7.2)
debug_inspector (>= 0.0.1)
bourbon (4.2.7)
sass (~> 3.4)
thor (~> 0.19)
brakeman (3.4.1)
breadcrumbs_on_rails (3.0.1)
builder (3.2.2)
bullet (4.14.4)
activesupport (>= 3.0.0)
uniform_notifier (>= 1.6.0)
bundler-audit (0.3.1)
bundler (~> 1.2)
thor (~> 0.18)
byebug (9.0.6)
capybara (2.7.1)
addressable
mime-types (>= 1.16)
nokogiri (>= 1.3.3)
rack (>= 1.0.0)
rack-test (>= 0.5.4)
xpath (~> 2.0)
capybara-webkit (1.11.1)
capybara (>= 2.3.0, < 2.8.0)
json
carrierwave (0.10.0)
activemodel (>= 3.2.0)
activesupport (>= 3.2.0)
json (>= 1.7)
mime-types (>= 1.16)
choice (0.2.0)
chronic (0.10.2)
code_analyzer (0.4.5)
sexp_processor
codeclimate-test-reporter (0.4.7)
simplecov (>= 0.7.1, < 1.0.0)
coderay (1.1.0)
coffee-rails (4.2.1)
coffee-script (>= 2.2.0)
railties (>= 4.0.0, < 5.2.x)
coffee-script (2.4.1)
coffee-script-source
execjs
coffee-script-source (1.11.1)
colored (1.2)
concurrent-ruby (1.0.2)
connection_pool (2.2.0)
countries (2.0.2)
i18n_data (~> 0.7.0)
money (~> 6.7)
unicode_utils (~> 1.4)
crack (0.4.2)
safe_yaml (~> 1.0.0)
database_cleaner (1.4.1)
debug_inspector (0.0.2)
decent_exposure (2.3.2)
deep_merge (1.1.1)
devise (3.5.6)
bcrypt (~> 3.0)
orm_adapter (~> 0.1)
railties (>= 3.2.6, < 5)
responders
thread_safe (~> 0.1)
warden (~> 1.2.3)
diff-lcs (1.2.5)
docile (1.1.5)
domain_name (0.5.24)
unf (>= 0.0.5, < 1.0.0)
dotenv (2.0.0)
dotenv-rails (2.0.0)
dotenv (= 2.0.0)
elasticsearch (1.0.12)
elasticsearch-api (= 1.0.12)
elasticsearch-transport (= 1.0.12)
elasticsearch-api (1.0.12)
multi_json
elasticsearch-model (0.1.7)
activesupport (> 3)
elasticsearch (> 0.4)
hashie
elasticsearch-rails (0.1.7)
elasticsearch-transport (1.0.12)
faraday
multi_json
email_spec (1.6.0)
launchy (~> 2.1)
mail (~> 2.2)
erubis (2.7.0)
excon (0.45.3)
execjs (2.7.0)
factory_girl (4.5.0)
activesupport (>= 3.0.0)
factory_girl_rails (4.5.0)
factory_girl (~> 4.5.0)
railties (>= 3.0.0)
faker (1.6.6)
i18n (~> 0.5)
faraday (0.9.1)
multipart-post (>= 1.2, < 3)
fast_stack (0.1.0)
rake
rake-compiler
fission (0.5.0)
CFPropertyList (~> 2.2)
flag-icons-rails (1.1.2)
sass (~> 3.2)
flamegraph (0.1.0)
fast_stack
fog (1.29.0)
fog-atmos
fog-aws (~> 0.0)
fog-brightbox (~> 0.4)
fog-core (~> 1.27, >= 1.27.4)
fog-ecloud
fog-json
fog-local
fog-powerdns (>= 0.1.1)
fog-profitbricks
fog-radosgw (>= 0.0.2)
fog-riakcs
fog-sakuracloud (>= 0.0.4)
fog-serverlove
fog-softlayer
fog-storm_on_demand
fog-terremark
fog-vmfusion
fog-voxel
fog-xml (~> 0.1.1)
ipaddress (~> 0.5)
nokogiri (~> 1.5, >= 1.5.11)
fog-atmos (0.1.0)
fog-core
fog-xml
fog-aws (0.1.2)
fog-core (~> 1.27)
fog-json (~> 1.0)
fog-xml (~> 0.1)
ipaddress (~> 0.8)
fog-brightbox (0.7.1)
fog-core (~> 1.22)
fog-json
inflecto (~> 0.0.2)
fog-core (1.30.0)
builder
excon (~> 0.45)
formatador (~> 0.2)
mime-types
net-scp (~> 1.1)
net-ssh (>= 2.1.3)
fog-ecloud (0.1.1)
fog-core
fog-xml
fog-json (1.0.1)
fog-core (~> 1.0)
multi_json (~> 1.0)
fog-local (0.2.1)
fog-core (~> 1.27)
fog-powerdns (0.1.1)
fog-core (~> 1.27)
fog-json (~> 1.0)
fog-xml (~> 0.1)
fog-profitbricks (0.0.2)
fog-core
fog-xml
nokogiri
fog-radosgw (0.0.4)
fog-core (>= 1.21.0)
fog-json
fog-xml (>= 0.0.1)
fog-riakcs (0.1.0)
fog-core
fog-json
fog-xml
fog-sakuracloud (1.0.1)
fog-core
fog-json
fog-serverlove (0.1.2)
fog-core
fog-json
fog-softlayer (0.4.5)
fog-core
fog-json
fog-storm_on_demand (0.1.1)
fog-core
fog-json
fog-terremark (0.1.0)
fog-core
fog-xml
fog-vmfusion (0.1.0)
fission
fog-core
fog-voxel (0.1.0)
fog-core
fog-xml
fog-xml (0.1.2)
fog-core
nokogiri (~> 1.5, >= 1.5.11)
foreman (0.82.0)
thor (~> 0.19.1)
formatador (0.2.5)
formtastic (3.1.4)
actionpack (>= 3.2.13)
formtastic_i18n (0.6.0)
formulaic (0.2.0)
activesupport
capybara
i18n
foundation-icons-sass-rails (3.0.0)
railties (>= 3.1.1)
sass-rails (>= 3.1.1)
foundation-rails (5.5.1.1)
railties (>= 3.1.0)
sass (>= 3.3.0, < 3.5)
friendly_id (5.1.0)
activerecord (>= 4.0.0)
fuubar (2.0.0)
rspec (~> 3.0)
ruby-progressbar (~> 1.4)
globalid (0.3.7)
activesupport (>= 4.1.0)
has_scope (0.6.0)
actionpack (>= 3.2, < 5)
activesupport (>= 3.2, < 5)
hashie (3.4.1)
heroku (3.32.0)
heroku-api (>= 0.3.19)
launchy (>= 0.3.2)
multi_json (>= 1.10)
netrc (>= 0.10.0)
rest-client (>= 1.6.0)
rubyzip (>= 0.9.9)
heroku-api (0.3.23)
excon (~> 0.44)
multi_json (~> 1.8)
http-cookie (1.0.2)
domain_name (~> 0.5)
i18n (0.7.0)
i18n_data (0.7.0)
inflecto (0.0.2)
inherited_resources (1.6.0)
actionpack (>= 3.2, < 5)
has_scope (~> 0.6.0.rc)
railties (>= 3.2, < 5)
responders
inline_svg (0.11.0)
activesupport (>= 4.0)
loofah (>= 2.0)
nokogiri (~> 1.6)
interactor (3.1.0)
ipaddress (0.8.0)
jquery-rails (4.0.4)
rails-dom-testing (~> 1.0)
railties (>= 4.2.0)
thor (>= 0.14, < 2.0)
jquery-ui-rails (5.0.5)
railties (>= 3.2.16)
json (1.8.3)
jsonapi (0.1.1.beta2)
json (~> 1.8)
jwt (1.5.0)
kaminari (0.17.0)
actionpack (>= 3.0.0)
activesupport (>= 3.0.0)
launchy (2.4.3)
addressable (~> 2.3)
letter_opener (1.3.0)
launchy (~> 2.2)
libv8 (3.16.14.15)
loofah (2.0.3)
nokogiri (>= 1.5.9)
mail (2.6.4)
mime-types (>= 1.16, < 4)
masonry-rails (0.2.4)
railties
memory_profiler (0.9.4)
metamagic (3.1.7)
rails (>= 3.0.0)
method_source (0.8.2)
mime-types (2.99.3)
mini_magick (4.2.4)
mini_portile2 (2.1.0)
minitest (5.10.1)
money (6.7.1)
i18n (>= 0.6.4, <= 0.7.0)
sixarm_ruby_unaccent (>= 1.1.1, < 2)
multi_json (1.12.1)
multi_xml (0.5.5)
multipart-post (2.0.0)
mysql (2.9.1)
net-scp (1.2.1)
net-ssh (>= 2.6.5)
net-ssh (2.9.2)
netrc (0.10.3)
newrelic_rpm (3.12.1.298)
nokogiri (1.6.8.1)
mini_portile2 (~> 2.1.0)
oauth2 (1.0.0)
faraday (>= 0.8, < 0.10)
jwt (~> 1.0)
multi_json (~> 1.3)
multi_xml (~> 0.5)
rack (~> 1.2)
oj (2.18.0)
omniauth (1.2.2)
hashie (>= 1.2, < 4)
rack (~> 1.0)
omniauth-facebook (2.0.1)
omniauth-oauth2 (~> 1.2)
omniauth-google-oauth2 (0.2.4)
omniauth (~> 1.0)
omniauth-oauth2 (~> 1.1)
omniauth-oauth2 (1.3.0)
oauth2 (~> 1.0)
omniauth (~> 1.2)
orm_adapter (0.5.0)
parser (2.3.3.1)
ast (~> 2.2)
pg (0.18.1)
pgbackups-archive (1.0.3)
fog-aws
heroku (>= 3.28.6, <= 3.32)
rake
polyamorous (1.3.1)
activerecord (>= 3.0)
powerpack (0.1.1)
pry (0.10.1)
coderay (~> 1.1.0)
method_source (~> 0.8.1)
slop (~> 3.4)
pry-rails (0.3.4)
pry (>= 0.9.10)
puma (3.6.2)
pundit (1.1.0)
activesupport (>= 3.0.0)
quiet_assets (1.1.0)
railties (>= 3.1, < 5.0)
rabl (0.13.1)
activesupport (>= 2.3.14)
rack (1.6.5)
rack-canonical-host (0.1.0)
addressable
rack (~> 1.0)
rack-cors (0.4.0)
rack-mini-profiler (0.10.1)
rack (>= 1.2.0)
rack-test (0.6.3)
rack (>= 1.0)
rails (4.2.7.1)
actionmailer (= 4.2.7.1)
actionpack (= 4.2.7.1)
actionview (= 4.2.7.1)
activejob (= 4.2.7.1)
activemodel (= 4.2.7.1)
activerecord (= 4.2.7.1)
activesupport (= 4.2.7.1)
bundler (>= 1.3.0, < 2.0)
railties (= 4.2.7.1)
sprockets-rails
rails-assets-autosize (3.0.17)
rails-assets-dropzone (4.3.0)
rails-assets-jquery (2.1.4)
rails-assets-jquery-bar-rating (1.1.0)
rails-assets-jquery (>= 1.7.2)
rails-assets-jquery-cascading-dropdown (1.2.5)
rails-assets-jquery-placeholder (2.1.1)
rails-assets-jquery (>= 1.6)
rails-assets-jquery-ui (1.11.4)
rails-assets-jquery (>= 1.6)
rails-assets-lite-uploader (2.2.0)
rails-assets-jquery (>= 1.8)
rails-assets-lodash (4.14.2)
rails-assets-microplugin (0.0.3)
rails-assets-pnotify (3.0.0)
rails-assets-jquery (>= 1.6)
rails-assets-rangeslider.js (1.2.1)
rails-assets-jquery (>= 1.9.0)
rails-assets-sachinchoolur--lightGallery (1.3.5)
rails-assets-jquery (>= 1.7.0)
rails-assets-selectize (0.12.3)
rails-assets-jquery (>= 1.7.0)
rails-assets-microplugin (~> 0.0.0)
rails-assets-sifter (~> 0.5.0)
rails-assets-sifter (0.5.2)
rails-assets-timepicker (1.7.0)
rails-assets-jquery (>= 1.7)
rails-deprecated_sanitizer (1.0.3)
activesupport (>= 4.2.0.alpha)
rails-dom-testing (1.0.7)
activesupport (>= 4.2.0.beta, < 5.0)
nokogiri (~> 1.6.0)
rails-deprecated_sanitizer (>= 1.0.1)
rails-erd (1.4.0)
activerecord (>= 3.2)
activesupport (>= 3.2)
choice (~> 0.2.0)
ruby-graphviz (~> 1.2)
rails-html-sanitizer (1.0.3)
loofah (~> 2.0)
rails_12factor (0.0.3)
rails_serve_static_assets
rails_stdout_logging
rails_best_practices (1.15.7)
activesupport
code_analyzer (>= 0.4.3)
colored
erubis
i18n
json
require_all
ruby-progressbar
rails_serve_static_assets (0.0.4)
rails_stdout_logging (0.0.3)
railties (4.2.7.1)
actionpack (= 4.2.7.1)
activesupport (= 4.2.7.1)
rake (>= 0.8.7)
thor (>= 0.18.1, < 2.0)
rainbow (2.1.0)
rake (12.0.0)
rake-compiler (0.9.5)
rake
ransack (1.8.2)
actionpack (>= 3.0)
activerecord (>= 3.0)
activesupport (>= 3.0)
i18n
polyamorous (~> 1.3)
react-rails (1.8.1)
babel-transpiler (>= 0.7.0)
coffee-script-source (~> 1.8)
connection_pool
execjs
railties (>= 3.2)
tilt
ref (2.0.0)
remotipart (1.2.1)
require_all (1.3.2)
responders (2.3.0)
railties (>= 4.2.0, < 5.1)
rest-client (1.8.0)
http-cookie (>= 1.0.2, < 2.0)
mime-types (>= 1.16, < 3.0)
netrc (~> 0.7)
rinku (1.7.3)
rollbar (2.13.3)
multi_json
rspec (3.5.0)
rspec-core (~> 3.5.0)
rspec-expectations (~> 3.5.0)
rspec-mocks (~> 3.5.0)
rspec-core (3.5.4)
rspec-support (~> 3.5.0)
rspec-expectations (3.5.0)
diff-lcs (>= 1.2.0, < 2.0)
rspec-support (~> 3.5.0)
rspec-mocks (3.5.0)
diff-lcs (>= 1.2.0, < 2.0)
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rspec-rails (3.5.2)
actionpack (>= 3.0)
activesupport (>= 3.0)
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rspec-expectations (~> 3.5.0)
rspec-mocks (~> 3.5.0)
rspec-support (~> 3.5.0)
rspec-support (3.5.0)
rubocop (0.46.0)
parser (>= 2.3.1.1, < 3.0)
powerpack (~> 0.1)
rainbow (>= 1.99.1, < 3.0)
ruby-progressbar (~> 1.7)
unicode-display_width (~> 1.0, >= 1.0.1)
ruby-graphviz (1.2.2)
ruby-progressbar (1.8.1)
rubyzip (1.1.7)
safe_yaml (1.0.4)
sass (3.4.22)
sass-rails (5.0.6)
railties (>= 4.0.0, < 6)
sass (~> 3.1)
sprockets (>= 2.8, < 4.0)
sprockets-rails (>= 2.0, < 4.0)
tilt (>= 1.1, < 3)
scss_lint (0.38.0)
rainbow (~> 2.0)
sass (~> 3.4.1)
seedbank (0.3.0)
sexp_processor (4.7.0)
shoulda-matchers (2.8.0)
activesupport (>= 3.0.0)
simple_form (3.1.0)
actionpack (~> 4.0)
activemodel (~> 4.0)
simplecov (0.9.2)
docile (~> 1.1.0)
multi_json (~> 1.0)
simplecov-html (~> 0.9.0)
simplecov-html (0.9.0)
sitemap_generator (5.2.0)
builder (~> 3.0)
sixarm_ruby_unaccent (1.1.1)
skim (0.10.0)
coffee-script
coffee-script-source (>= 1.2.0)
slim (>= 3.0)
sprockets (>= 2, < 4)
slim (3.0.3)
temple (~> 0.7.3)
tilt (>= 1.3.3, < 2.1)
slim-rails (3.0.1)
actionmailer (>= 3.1, < 5.0)
actionpack (>= 3.1, < 5.0)
activesupport (>= 3.1, < 5.0)
railties (>= 3.1, < 5.0)
slim (~> 3.0)
slop (3.6.0)
sprockets (3.7.0)
concurrent-ruby (~> 1.0)
rack (> 1, < 3)
sprockets-rails (3.2.0)
actionpack (>= 4.0)
activesupport (>= 4.0)
sprockets (>= 3.0.0)
stackprof (0.2.7)
temple (0.7.5)
test_after_commit (0.4.1)
activerecord (>= 3.2)
therubyracer (0.12.2)
libv8 (~> 3.16.14.0)
ref
thor (0.19.4)
thread_safe (0.3.5)
tilt (2.0.5)
truncate_html (0.9.3)
tzinfo (1.2.2)
thread_safe (~> 0.1)
uglifier (2.7.2)
execjs (>= 0.3.0)
json (>= 1.8.0)
unf (0.1.4)
unf_ext
unf_ext (0.0.7.1)
unicode-display_width (1.1.1)
unicode_utils (1.4.0)
uniform_notifier (1.8.0)
validate_url (1.0.0)
activemodel (>= 3.0.0)
addressable
warden (1.2.6)
rack (>= 1.0)
web-console (2.1.3)
activemodel (>= 4.0)
binding_of_caller (>= 0.7.2)
railties (>= 4.0)
sprockets-rails (>= 2.0, < 4.0)
webmock (1.21.0)
addressable (>= 2.3.6)
crack (>= 0.3.2)
xpath (2.0.0)
nokogiri (~> 1.3)
PLATFORMS
ruby
DEPENDENCIES
active_link_to
active_model_serializers
activeadmin!
activerecord-postgres-earthdistance
addressable
autoprefixer-rails
awesome_print
brakeman
breadcrumbs_on_rails
bullet
bundler-audit
byebug
capybara
capybara-webkit
carrierwave
chronic
codeclimate-test-reporter
coffee-rails
countries
database_cleaner
decent_exposure
deep_merge
devise
dotenv-rails
elasticsearch-model
elasticsearch-rails
email_spec
factory_girl_rails
faker
flag-icons-rails
flamegraph
fog
fog-aws
foreman
formulaic
foundation-icons-sass-rails
foundation-rails
friendly_id
fuubar
inline_svg
interactor
jquery-rails (= 4.0.4)
jquery-ui-rails (= 5.0.5)
kaminari
launchy
letter_opener
masonry-rails
memory_profiler
metamagic
mini_magick
mysql
newrelic_rpm
oj
omniauth
omniauth-facebook
omniauth-google-oauth2
pg
pgbackups-archive
pry-rails
puma
pundit
quiet_assets
rabl
rack-canonical-host
rack-cors
rack-mini-profiler
rails (= 4.2.7.1)
rails-assets-autosize!
rails-assets-dropzone!
rails-assets-jquery-bar-rating!
rails-assets-jquery-cascading-dropdown!
rails-assets-jquery-placeholder!
rails-assets-jquery-ui!
rails-assets-lite-uploader!
rails-assets-lodash!
rails-assets-pnotify!
rails-assets-rangeslider.js!
rails-assets-sachinchoolur--lightGallery!
rails-assets-selectize!
rails-assets-timepicker!
rails-erd
rails_12factor
rails_best_practices
react-rails
remotipart
responders
rinku
rollbar
rspec-rails
rubocop
sass-rails
scss_lint
seedbank
shoulda-matchers
simple_form
sitemap_generator
skim
slim
slim-rails
stackprof
test_after_commit
therubyracer
truncate_html
uglifier
validate_url
web-console
webmock
RUBY VERSION
ruby 2.3.1p112
BUNDLED WITH
1.13.6
双方的价值。
答案 1 :(得分：0)
向您的类构造函数添加显式，错误将返回。
explicit foo(int i)
编译器会发生什么，请尝试以下代码：
if (divisor == 0)
通过从0创建foo对象（它可以这样做，因为你的构造函数不是显式的），但是失败了，因为比较了两个foo对象，你没有提供这个操作符。
添加功能时：
friend bool operator==(const foo<t>& lhs, const foo<t>& rhs) {
// do some comparison here
}
您确实提供了必要的操作员。